18 SEXUAL DIMORPHISM 



facts of development throughout the animal kingdom has 

 shown that this law is bj no means universal It is true 

 in regard to a certain number of facts in the development 

 of the higher vertebrates, but it is not the whole truth 

 about them, and it is contradicted by a great many other 

 facts even in the development of reptiles, birds, and 

 mammals. For example, snakes are characterised by the 

 absence of limbs. In some snakes rudiments of the hind 

 limbs are present in the adult condition, but in no snake 

 has any trace of the fore limbs been discovered in any 

 embryonic stage. Yet we cannot doubt that the ancestors 

 of snakes possessed two pairs of limbs. Again, there can 

 be no doubt that the wing of the bird has been evolved 

 from a limb with five digits like that of many reptiles, but 

 the wing contains only three digits, and the most complete 

 embryological investigation has only succeeded in discover- 

 ing small and very doubtful rudiments of the lost two. 

 The ancestors of birds had teeth, but no trace of teeth 

 has been found in the embryo. In the horse, again, there 

 are traces of the 2nd and 4th metacarpals and metatarsals 

 in the limbs of the adult, but examination of the development 

 has shown that only the merest vestiges of the 2nd and 4th 

 digits are ever formed, and of the 1st and oth none at all 



Balfour has expressed the general results of observation 

 in the statement that ancestral stages are liable to be omitted 

 from embryonic development by abbreviation, to be obscured 

 or replaced by new characters in free larval development. 

 He also suggested that the retention of the branchial arches 

 and clefts in the embryos of higher vertebrates was due to the 

 fact that these structures were functional in the larval stage 

 of the amphibian ancestors of these vertebrates after they 

 had become rudimentary in the adults, and that the limbs in 

 snakes had completely disappeared because there was no 

 such advantage in their retention at a particular stage. 



