ABSTRACT
“The same experiment can be tried with the embryos of animals. Take the ovule of the worm, the eagle, the elephant, and of man himself. Let the most skilled observer apply the most searching tests to distinguish the one from the other, and he will fail. One of the difficulties attending the study of “species” among the higher forms of plants and animals has always been the length of time required to obtain any large number of generations on which to make observations. Doubtless these forms never multiply at this rate uninterruptedly for any great length of time, or else they would occupy the whole world to the exclusion of every other form of life. Plants were known to be composed of cells, and their cells were studied and described some years before it was understood that animals also are composed of cells as units.
