76 Evolution and Adaptation 



result is in exact harmony with the well-known law of Von 

 Baer, which refers to natural species, both nearly related and 

 widely dissimilar. Von Baer's observations as well as Dar- 

 win's, and as well as those of every student who has ever 

 compared the embryos of two vertebrate species, may be 

 summarized as follows : — 



" Animals which, though related, are very similar in the 

 adult state, resemble each other more closely in early stages 

 of development, often, indeed, so closely as to be indistin- 

 guishable in those early stages. As development proceeds 

 in such species, the differences between the two embryos com- 

 pared become more and more pronounced." On this point, 

 which is an essential one, I cannot agree with Hurst ; for I 

 do not think that the facts show that the early stages of two 

 related forms are necessarily more and more alike the farther 

 back we go. The resemblance that is sometimes so striking in 

 the earlier stages is due to the fewer points there are for 

 comparison, and to the less development of the parts then 

 present. Hurst continues : " If similar comparisons could be 

 instituted between the ancestral species and its much modi- 

 fied descendants, there is no reason for doubting that a similar 

 result would be reached. This, indeed, has been done in the 

 case of some breeds of pigeons, which we have excellent 

 reasons for believing to be descended from Columba livia. 

 True, C. livia is not a very remote ancestor, but I do not 

 think that will vitiate the argument. Let me quote Darwin 

 verbatim : ' As we have conclusive evidence that the breeds 

 of the pigeon are descended from a single wild species, I 

 have compared the young within twelve hours after being 

 hatched ; I have carefully measured the proportions (but will 

 not here give the details) of the beak, width of mouth, length 

 of nostril, and of eyelid, size of feet, and length of leg in 

 the wild, parent species, in pouters, fantails, runts, barbs, 

 dragons, carriers, and tumblers. Now some of these birds 

 when mature differ in so extraordinary a manner in the 



