CHAPTER V. 



THE ARGUMENTS FROM EMBRYOLOGY. 



$ 123. THERE was briefly set forth in 52, a remarkable 

 induction established by Yon Baer ; who " found that in its 

 earliest stage, every organism has the greatest number of 

 characters in common with all other organisms in their 

 earliest stages ; that at a stage somewhat later, its structure 

 is like the structures displayed at corresponding phases by a 

 less extensive multitude of organisms ; that at each subse- 

 quent stage, traits are acquired which successively distin- 

 guish the developing embryo from groups of embryos that it 

 previously resembled thus step by step diminishing the 

 class of embryos which it still resembles ; and that thus the 

 class of similar forms is finally narrowed to the species of 

 which it is a member." Though this generalization is to be 

 taken with qualifications, yet, as an average truth, it may 

 be regarded as beyond question ; and as an average truth, it 

 has a profound significance. 



For if we follow out in thought the implications 

 of this truth if we conceive the germs of all kinds 

 of organisms simultaneously developing ; if after taking 

 their first step together, we imagine at the second step, one 

 half of the vast multitude diverging from the other half; if, 

 at the next step, we mentally watch each of these great 

 assemblages beginning to take two or more routes of 

 development ; if we represent to ourselves this bifurcation 

 simultaneously going on, stage after stage, in all the 



