ABSTRACT
T H E N A T U R E OF L I F E beyond the detail of the working of one kind of machine to a general picture of the whole range of such mechanisms, we can in this case easily see in what general framework the picture can be arranged. All these things-cars, buses, tractors, aeroplanes, ships and so on-—have been designed and made by men for particular purposes. They can be ordered in terms of the particular human purposes they were designed to meet.Admittedly some odd questions will arise. For instance, why is the engine of the ordinary motor car in front, although the drive is applied to the back wheels? The answer is probably at least in part historical, related to the derivation of automobiles from horse-drawn carriages, an evolutionary process whose traces are easy to see in the carriage work of the earliest motor cars. Again we shall find that some features of these mechanisms have acquired their particular character from the requirements of the manufacturing process; certain types of shape of body, for instance, are easier to produce by a mechanical press than others. But if we were to attempt to give an orderly account of man-made self-propelled mechanisms, these questions of historical derivation and of the requirements of manufacturing processes play a comparatively small part in comparison with the influence of the purposes which animated the designer.When we turn from the mechanisms which man fabricates to those which we find around us as natural living organisms, we find ourselves faced with the same problem of giving a general account of an enormous range of different operating systems, and therefore with the same need to take a broader view than the detailed understanding of how any one system works. We also find ourselves confronted with similar phenomena of historical derivation-in this case the processes of organic evolution; and also with phenomena comparable to those of manufacture-—the development of the organism from a fertilized germ to a fully differentiated adult. But there is no human designer of the natural world of living things. We can postulate a super-human designer, a creating God. If he is conceived of as outside the Universe, then his purposes are not open to our understanding as are those of the human designers of vehicles, and it becomes senseless to try to use them as a framework for a general
I N N A T E P O T E N T I A L I T I E S rational understanding of the world as we find it. On the other hand, if the Creator’s purposes are expressed in the world, then we have to examine it to discover what they are, since we have no possibility of interrogating him as we can interrogate a human designer. Thus, an appeal to a purposeful Creator as an explanation for the nature of living things either abolishes the possibility of rational biology, or leaves us just where we were before, faced with the need to account for the phenomena of life in terms of the happenings which we can see preceding in front of us.It is convenient to classify the processes by which living things come into being into the three categories of development, heredity and evolution. For the reasons just stated, processes of these kinds play an enormously more important part in the theory of biology than they do in the theory of man-made mechanisms or of the inorganic world as a whole. It is perhaps the most characteristic feature of biology, and its greatest point of difference from the sciences of physics and chemistry, that it deals with entities which must be envisaged simultaneously on four different time scales. Not only must we study the hour-to-hour or minute-to-minute operations of living things as going concerns, but we cannot leave out of account the slower processes, occurring in period of time comparable to a lifetime, by which the egg develops into the grown-up adult, and finally towards senescence and death. On a longer time scale again, there are phenomena which must be measured in terms of a small number of lifetimes; they are the processes of heredity, by which characteristics of organisms are passed on from parent to offspring. Finally, on the time-scale of many hundreds of generations, there are the slow processes of evolution, by which the character of the individuals in a given population gradually changes, and the population may become split up into two or more different species. Biological theory, which has to cover adequately all four of these time-scales, is of necessity much more complex than that of the physical sciences.It is a rather atomistic insight which provides perhaps the best Ariadne’s thread for those who would penetrate the intricacies of this biological maze. The first principle of an atomistic
T H E N A T U R E OF L I F E approach to a problem is to try to catch the entities concerned in their simplest terms. From the time when it is no more than a newly fertilized egg, the mouse is just as much a mouse as the creature you may see disappearing in a hole in the wainscotting; and the human egg is as just as much a man (though perhaps not as much of a man) as you are. We have, then, in the fertilized egg something relatively simple and yet comprehensive enough to include, in some way or other, the whole system of a living organism. Understanding of the egg therefore provides the leading clue to the elucidation of the whole complexity of the biological realm.The fertilized egg, when inspected with the microscope, consists of no more than a small lump of living material which contains two other smaller masses which are in process of fusing together. Living material only occurs in the form of small lumps, which are bounded by some sort of a membrane. Each such lump is known as a cell, and the fertilized egg is also a cell of a particular type. The material which makes up the main body of the cell is known as the cytoplasm. In it there is embodied a smaller mass of a special kind which is known as the nucleus. In the newly fertilized egg the cytoplasm contains at first two such masses, one being the nucleus which was formed in the egg cell as it grew in the body of the mother, and the other being a nucleus which was brought into the egg by the sperm from the father. These two nuclei very soon fuse together to form a single one. The whole body of the newly developing animal will be formed by the gradual multiplication of the egg cell and its contained nucleus; this will grow till it develops into a larger mass made up of innumerable small cells, each containing cytoplasm in which a nucleus is embedded.It is clear that the character of a newly fertilized egg depends on the nature of the parents from which it was produced. Acock and hen will never give rise to an egg from which a duck will develop. A clue to the understanding of the fertilized egg therefore lies in the study of heredity. The biological discipline concerned with this, usually known nowadays as Genetics, has therefore a key position in the whole edifice of biological theory.*In spite of its importance, genetics is one of the most re
I N N A T E P O T E N T I A L I T I E S cently developed of all the major branches of biology. It was only at the beginning of this century that the nature of the basic principles governing the phenomena of heredity was realized by biologists in general. However, in the sixty years that have elapsed since then, genetics has grown to be one of the most fully developed and logically coherent parts of biological theory. An examination of how it has come into flower, and of the questions which still confront it, will provide a good example of the mutually reinforcing interplay of atomistic and continuum theories which is so characteristic of biology.The first step in the understanding of heredity is to realize that what a pair of parents donate to their offspring is a set of potentialities, not a set of already formed characteristics. You do not inherit fair or dark hair, blue or brown eyes, from your parents; what you inherit is something which endows you with the capacity for developing eyes of a particular colour under certain particular circumstances, and perhaps a different colour under other circumstances. Even if your parents were both Anglo-Saxons you do not inherit their white skin; you inherit potentialities of such a kind that if you grow up with very weak sunlight your skin will be very fair in colour, while if you are frequently exposed to much stronger sun it will be considerably browner. Nowadays we use the word 'genotype' for the collection of potentialities which are inherited. Contrast this with the 'phenotype', which is the name for the collection of characteristics which an individual actually develops under the particular circumstances in which he happens to grow up. Any one genotype may give rise to many somewhat different phenotypes, corresponding to the different environments in which development occurs.In the pre-history of genetics-before, that is to say, its development as an orderly science got under way-the genotype was envisaged as having a character roughly comparable to that of a fluid. When two animals mated and produced offspring the hereditary contributions of the two parents were supposed to blend in the newborn creature. In fact the hereditary contribution was often spoken of somewhat metaphorically as 'blood', so that people spoke of animals belonging to certain 'blood lines',
FIG. 1 The two drawings on the left show the old ‘blending' theory of heredity; a sperm (dark) unites with an egg (pale) and their characters blend to give an offspring of intermediate type. On the right is the modern theory. The egg contains a nucleus, in which are shown two chromosomes, a V-shaped one and a long rod, on which, at one particular place, is a hereditary factor or gene indicated by a circle. The sperm also contains two similar chromosomes, the rod-chromosome containing a slightly different form of the gene, indicated by a cross. The offspring contains both these different kinds of genes, which come into the same nucleus but do not fuse or blend.
