TIIE ARGUMENTS FROM EMBRYOLOGY. 367 



therefore necessitated a morphological unity in their primitive 

 states ; there arises the obvious answer, that the morphologi- 

 cal unity thus implied, is not the only morphological unity 

 to be accounted for. Were this the only unity, the various 

 kinds of organisms, setting out from a common primordial 

 form, should all begin from the first to diverge individually, 

 as so many radii from a centre ; which they do not. If, other- 

 wise, it be said that organisms were framed upon certain 

 types, and that those of the same type continue developing 

 together in the same direction, until it is time for them to 

 begin putting on their specialities of structure ; then, the 

 answer is, that when they do finally diverge, they ought 

 severally to develop in direct lines towards their final forms. 

 No reason can be assigned why, having once parted company, 

 some should progress towards their final forms by irregular 

 or circuitous routes. On the hypothesis of design, such de- 

 viations are inexplicable. 



The hypothesis of evolution, however, while it pre- supposes 

 those general relations among embryos which are found tc 

 exist, also affords explanations of these minor nonconformities. 

 If, as any rational theory of evolution pre-supposes, the pro- 

 gressive differentiations of organic forms from one another 

 during past times, have resulted, as they are resulting still, 

 from the direct and indirect effects of external conditions 

 if organisms have become different, either by immediate 

 adaptations to unlike habits of life, or by the mediate adapta- 

 tions resulting from preservation of the individuals most 

 fitted for such habits of life, or by both ; and if the embryonic 

 changes are related to the changes that were undergone by 

 ancestral races ; then these irregularities 'must be expected. 

 For the successive changes in modes of life pursued by 

 successive ancestral races, can have had no regularity of 

 sequence. In some cases they must have been more numerous 

 than in others ; in some cases they must have been greater 

 in degree than in others ; in some cases they must have been 

 to lower modes, in some cases to higher modes, and in some 



