Mutations and Evolution. 
241 
arterial system, a cartilaginous endo-skeleton, oro-nasal grooves 
and a notochord. But he finds “ equally important differences.” 
Nevertheless, he admits the essential point for recapitulation— 
that embryos pass through “ stages of structure permanent in 
lower members of the same group.” He also says (p. 43), “The 
evidence seems to indicate that in a number of cases adult variations 
of any part are accompanied by precedent similar alteration of 
the same part in the embryo.” We have already seen that this is 
accounted for on a mutational basis. 
It is well-known that Hyatt showed with fossil Ammonites 
that there is recapitulation in successive coils of the shell, the first 
coils often reproducing characters belonging to types known from the 
palaeontological record to be ancestral. His law of acceleration in 
development, deduced from purely palaeontological observations is 
simply another expression of the more recent embryological law 
of tachygenesis. 
Morgan (1916, p. 19) has recently expressed the view that the 
new mutationist ideas have played havoc with the biogenetic law. 
He says, for example, that the chick, the fish, and man all possess 
gill-slits at an early stage of their development merely because 
they have not lost them. But this merely glosses over a difficulty 
without explaining it. The fallacy in such an explanation is evident 
enough if one applies it to such a recent recapitulation as that of 
the parasitic Copepod, Achtheres (see page 232). In such a case the 
larval stage of this copepod corresponds obviously to the adult 
stage of free-living copepods. Achtheres, however, passes through 
and beyond this stage, and its adult stage has lost practically all its 
copepod features. We venture to think this is a recapitulatory 
phenomenon, involving a lengthening of the life-cycle and 
probably also the inheritance of acquired characters. 
Relation to Geographic Distribution. 
Brief allusion only will be made to this subject. We have 
already seen that recapitulatory characters appear to involve a 
gradual adaptation to a new habitat, while mutations do not. In 
this connection we have pointed out (Gates, 1917c) another 
relationship between variation and geographic distribution. With 
reference particularly to North American owls (Otus asio), it was 
shown that in Eastern North America the red colour phase, which 
occurs usually in the same regions as the gray with which it inter¬ 
breeds, probably originated as a mutation and behaves as a dominant 
Mendelian character. In Western America, on the other hand, a 
