472 THE GENERAL PRINCIPLES OF ZOOLOGY 



In many groups of animals we find structures which have 

 very different functions but are all constructed on a common 

 plan or type which has undergone the most varied modifica- 

 tions. Thus the front legs of the frog and of the leather-back. 

 turtle, the wings of the bird and of the bat, the flippers of the 

 seal and of the whale, the fore leg of the ox and the arm of 

 a man, are all built on the same plan of skeletal structure. The 

 parts of the vertebrate brain are essentially the same in all 

 classes, but the degree of differentiation which is found in the 

 higher mammals over the structure of the brain of a fish is 

 almost inconceivably great. Such facts as these, which demon- 

 strate a common type with manifold variations, point strongly 

 to a common descent. 



Still another argument for the theory of evolution is furnished 

 by anatomy in the presence of numerous structures, especially 

 in the higher animals, which are of no apparent use to the in- 

 dividual ; they are very rudimentary as compared with the 

 corresponding structures in lower animals where they are 

 actively functional. We note, as a few examples, the minute 

 hind limbs present in the python ; the very rudimentary and 

 useless wings of the Apteryx, a bird of New Zealand ; the rudi- 

 mentary wings of many beetles and other insects; the blind 

 eyes of cave animals ; and of the numerous rudimentary struc- 

 tures in man we may mention the minute tail, which never 

 pierces the skin, the hair of the general surface of the bodv, and 

 the vermiform appendix, which has not only become useless but 

 even dangerous. We have already referred to such structures 

 in connection with the Darwinian theory. Their presence is 

 readily comprehensible on the theory of evolution, but unintel- 

 ligible otherwise. 



Sexual dimorphism, so highly developed in man)- groups of 

 animals and very conspicuous among some birds and mammals, 

 where the two sexes are often so different as to suggest their 

 belonging to different species or even genera, affords an excel- 

 lent example of evolution, each sex having developed along lines 

 which best adapt it to its special activities. 



Embryology furnishes numerous facts which indicate the cor- 

 rectness of the theory of evolution. The simplest form ot ani- 

 mal life is a single cell; then we find a colony of nearly similar 



