“Our next aim, and with the same object, namely, its bearing upon the question of origin, will be the study of the young, the collecting of eggs and embryos. This is the more important, since museums generally show only adult specimens. As far as I know, the Zoölogical Museum at Cambridge is the only one containing large collections of embryological specimens from all the classes of the animal kingdom. One significant fact, however, is already known. In their earliest stages of growth all animals of the same class are much more alike than in their adult condition, and sometimes so nearly alike as hardly to be distinguished. Indeed, there is an early period when the resemblances greatly outweigh the differences. How far the representatives of different classes resemble one another remains to be ascertained with precision. There are two possible interpretations of these facts. One is that animals so nearly identical in the beginning must have been originally derived from one germ, and are but modifications or transmutations, under various physical conditions, of this primitive unit. The other interpretation, founded on the same facts, is, that since, notwithstanding this material identity in the beginning, no germ ever grows to be different from its parent, or diverges from the pattern imposed upon it at its birth, therefore some other cause besides a material one must control its development; and if this be so, we have to seek an explanation of the differences between animals outside of physical influences. Thus far both these views rest chiefly upon personal convictions and opinions. The true solution of the problem must be sought in the study of the development of the animals themselves, and embryology is still in its infancy; for, though a very complete study of the embryology of a few animals has been made, yet these investigations include so small a number of representatives from the different classes of the animal kingdom that they do not yet give a basis for broad generalizations. Very little is known of the earlier stages in the formation of hosts of insects whose later metamorphoses, including the change of the already advanced larva, first to the condition of a chrysalis and then to that of a perfect insect, have been carefully traced. It remains to be ascertained to what extent the caterpillars of different kinds of butterflies, for instance, resemble one another during the time of their formation in the egg. An immense field of observation is open in this order alone.