1898] SPECIES, SEX, AXD INDIVIDUAL 235 



individual in its development is compelled to climb its own genea- 

 logical tree. 



A more comprehensive and more accurate study of the facts of 

 development throughout the animal kingdom has shown that this 

 law is by 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 discovering 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 second and fourth metacarpals 

 and metatarsals in the limbs in the adult, but examination of the 

 development has shown that only the merest vestiges of the second 

 .and fourth digits are ever formed, and of the first and fifth none 

 at all. 



Balfour has expressed the general result of observation in the 

 statement that ancestral stages are liable to be omitted from em- 

 bryonic development by abbreviation, and 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 embryo 

 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 bad 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. 



Mr Sedgwick has lately elaborated this last suggestion into the 

 general theory that ancestral characters are only retained in the 

 embryo when the ancestral condition was once a larval condition 

 which has more recently become embryonic, in consequence of the 

 retention of the larva in the egg or within the body of the mother 

 until after its metamorphosis. 



These are numerous instances of the truth of Mr Sedgwick's 

 theory, but it is not a general theory of individual development. 

 The general theory will be found to correspond to that which I 

 have indicated in the case of the structural differences between 

 groups of species and between individual types in the same species. 



