100 Mutations and Evolution. 



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 emhryos 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 numher 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 

 palfeontological 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 91). 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 



