xv THE PHILOSOPHY OF ZOOLOGY 645 



of the Horse, of essentially the same bony elements. More difficult 

 still would it be to explain the cases in which what is a 

 functionally active and important part in one animal is to be found- 

 though only as a mere vestige, apparently quite useless in an 

 allied form. Very many instances of this phenomenon will be 

 found in the previous chapters. The wing of the Pigeon is an efficient 

 organ of flight ; in the New Zealand Kiwi or Apteryx it is a 

 vestige, not visible externally, being covered over by the feathers 

 and wholly without function ; yet this vestige possesses essentially 

 the same bony framework and the same muscles as the complete 

 and functional wing of the Pigeon. Again, the teeth of the 

 Rabbit are parts essential to the welfare and the very existence of 

 the animal, and persist throughout life ; while in the Whalebone- 

 Whale teeth are indeed developed in the foetal condition, but are 

 thrown off before or shortly after birth, never being of any use for 

 mastication or any other purpose. The conclusion that seems to 

 follow from these facts is that it is at least highly probable that the 

 Kiwi has vestiges of wings because it is descended from birds 

 which, like the Pigeon, possessed functionally useful wings ; and 

 that the Whalebone- Whale has teeth in the foetal state because 

 it is descended from ancestors which possessed teeth in the adult 

 condition. 



The fact that the embryos of animals of one great phylum or 

 class present a great resemblance to one another, and that, the 

 nearer the adult forms are in structure, the closer, usually, is the 

 similarity in their developmental stages, tells strongly in favour of 

 a theory of common descent. Thus the nauplius-stage is found 

 in a considerable number of groups of Crustacea, but it is only 

 between members of families whose structure is closely similar 

 that there is a very near correspondence in the precise character of 

 the nauplius and in the stages which the larva subsequently 

 passes through. 



Evidence of an allied character is afforded by the fact that in 

 the course of its development one of the higher animals some- 

 times appears to exhibit in successive stages features which are 

 permanent in forms lower in the scale. Thus the embryo of a 

 Mammal presents at an early stage visceral arches and clefts com- 

 parable to the branchial arches and clefts of a Fish, and has a 

 blood-circulation in accordance with this ; while at a later stage it 

 exhibits in these particulars some resemblance to an Amphibian, 

 later on to a Reptile, and only when development is further 

 advanced takes on its special Mammalian characters. Again, we 

 have seen that such an Amphibian as the Frog is, in its early 

 condition as a tadpole, to all intents and purposes a Fish. Such 

 phenomena may be explained, according to the theory of evolution, 

 by the supposition that the successive stages in the development 

 of the individual animal tend to reproduce, though in a very 



