ABSTRACT
As a rule, parasites are smaller and less complex organisms than their hosts, so that the natural relationships of the hosts might be expected to be better understood than those of their parasites—in which case we should look to the classification of the hosts to clarify the affinities of the parasites rather than vice versa. In recent times, much publicity has been given to parasites as indicators of the relationships of their hosts. Most parasites, among both plants and animals, are more or less restricted in their choice of hosts—a given species of parasite occurring naturally only in members of a particular genus, family or some such classificatory group. The term parasitism is often not considered to include the relationship between a phytophagous insect and its food-plant, but in fact there are considerable analogies between this relation and that of fleas, lice, helminths and so on to their vertebrate hosts.
