xv THE PHILOSOPHY OF ZOOLOGY 609 



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 sometimes 

 appears to exhibit in successive stages features which are per- 

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

 Mammal presents at an early stage visceral arches and clefts 

 comparable to the branchial arches and cleft's 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 Amphi- 

 bian, 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 ab- 

 breviated and often greatly modified shape, the stages through 

 which the group to which the animal belongs has passed in the 

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