Von Baer’s statement appears to be erroneous from a modern point of view in the following respects. We know that in certain large groups some forms develop in a very different way from that followed by other members of the group, as shown by the cephalopods, for instance, in the group of mollusks. Again, it is entirely arbitrary to assume that the group-characters are the first to appear, and then successively those of the order, family, genus, species. Finally, as has been said above, we do not find the early embryos of a group identical; for with a sufficient knowledge of the development it is always possible to distinguish between the embryos of different species, as well as between the adults, only it is more difficult to do so, because the embryonic forms are simpler. The most fundamental difference between the view of Von Baer and modern views is due to our acceptation of the theory of evolution which seems to make it possible to get a deeper insight into the meaning of the repetition, that carries us far ahead of Von Baer’s position. For with the acceptance of this doctrine we have an interpretation of how it is possible for the embryonic stages of most members of a group to have the same form, although they are not identical. There has been a continuous, although divergent, stream of living material, carrying along with it the substance out of which the similar embryonic forms are made. As the stream of embryonic material divided into different paths it has also changed many of the details, sometimes even all; but nevertheless it has often retained the same general method of development that is associated with its particular composition. We find the likeness, in the sense of similarity of plan, accounted for by the inheritance of the same sort of substance; the differences in the development must be accounted for in some other way.
Among modern writers Hurst alone has advanced a view that is similar in several respects to that which I have here defended. It may be well to give his statement, since it brings out certain points of resemblance with, as well as certain differences from, my own view.[11] He says: “Direct observation has shown that, when an animal species _varies_ (_i.e._ _becomes_ unlike what it was before) in adult structure, those stages in the development which are nearest the adult undergo a similar, but usually smaller, change. This is shown in domestic species by the observations of Darwin, and the result is in exact harmony with the well-known law of Von Baer, which refers to natural species, both nearly related and widely dissimilar. Von Baer’s observations as well as Darwin’s, and as well as those of every student who has ever compared the embryos of two vertebrate species, may be summarized as follows:—