dog depart from the common mode, but they remain alike up to a far later
stage than the one in which they are similar to the embryos of rats and
sheep. The rat and squirrel and rabbit, on their part, remain together
until long after they take leave of the carnivora and ungulates; while the
sheep and cattle and pigs have their own branch line, which they follow in
company after leaving the embryos of the other orders. The reasons for
these facts seem to be that the members of the three orders exemplified
have evolved from the same stock, which accounts for their embryonic
similarity for a long time after they collectively come to differ from
amphibia and reptiles, while the members in each order became
differentiated only later, wherefore their embryonic paths coincide for a
longer period. Thus the degree of adult resemblance which indicates the
closeness of relationship corresponds with the degree of embryonic
agreement; that is, the cat and dog are much alike and their modes of
development are essentially the same to the latest stages, while the cat
and horse agree only during the earliest and middle stages, and their lines
diverge before those of the cat and dog on the one hand, or those of the
horse and pig on the other.

* * * * *

Like the fundamental principle of comparative anatomy in its sphere, the
Law of Recapitulation, formulated as a summary description of the
foregoing and similar facts, is one that holds true throughout the entire
range of embryology and for every division of the animal series, however
large or small. We have discussed its broader application, and now we may
take up some of the more or less special cases mentioned in the earlier
section of the present chapter, to see how it may work in detail.

The flounder was noted as a variant of the fish theme which seemed to be a