At the end of the last century Weismann injected a new idea into our views concerning the origin of variations. He urged that variations are germinal, i.e. they first appear in the egg and the sperm as changes that later bring about modifications in the individual. The idea has been fruitful and is generally accepted by most biologists today. It means that the
offspring of a pair of animals are not affected by the structure or the activities of their parents, but the germ plasm is the unmodified stream from which both the parent and the young have arisen. Hence their resemblance. Now, it has been found that a variation arising in the germ plasm, no matter what its cause, may affect any stage in the development of the next individuals that arise from it. There is no reason to suppose that such a change produces a new character that always sticks itself, as it were, on to the end of the old series. This idea of germinal variation therefore carried with it the death of the older conception of evolution by superposition.
In more recent times another idea has become current, mainly due to the work of Bateson and of de Vries—the idea that variations are discontinuous. Such a conception does not fall easily into line with the statement of the biogenetic "law"; for actual experience with discontinuous variation has taught us that new characters that arise do not add themselves to the end of the line of already existing characters but if they affect the adult characters
they change them without, as it were, passing through and beyond them.
| Fig. 8. Diagram of head of chick A and B, showing gill slits, and aortic arches; and head of fish C showing aortic arches. (After Hesse.) | Fig. 9. Human embryo showing gill slits and aortic arches. (After His; from Marshall.) |
I venture to think that these new ideas and this new evidence have played havoc with the biogenetic "law". Nevertheless, there is an interpretation of the facts that is entirely
compatible with the theory of evolution. Let me illustrate this by an example.
Fig. 10. Young fish, dorsal view, and side view, showing gill slits. (After Kopsch.)
The embryos of the chick (fig. 8) and of man (fig. 9) possess at an early stage in their development gill-slits on the sides of the neck like those of fishes. No one familiar with the relations of the parts will for a moment doubt that the gill slits of these embryos and of the fish represent the same structures. When we look further into the matter we find that young fish also possess gill slits (fig. 10 and 11)—even in young stages in their development. Is it not