340 THE STUDY OF ANIMAL LIFE CHAP. 



illustrate the different kinds of evidence, and that under 

 three heads physiological, structural, and historical. 



Physiological. - - Living creatures are variable from 

 generation to generation. Even in the short span of a 

 human life transformations may be observed to be in 

 process. The ' mutations ' thrown off by some of the 

 evening primroses, and especially by (Enothera lamarckiana, 

 are both numerous and striking, and give us a vivid 

 impression of the copiousness of the fountain of change 

 that sometimes wells up in living creatures. One of the 

 most striking cases in the Natural History Collection of 

 the British Museum is that near the entrance, where on a 

 tree are perched domesticated pigeons of many sorts- 

 fantail, pouter, tumbler, and the like while in the centre 

 is the ancestral rock-dove, Colwnba lima, from which we 

 know that all the rest have been derived. In other 

 domesticated animals, and in cultivated plants, like the 

 many different forms of cabbage, we may find similar 

 direct evidence of evolution. But what occurs under 

 man's supervision in the domestication of animals and in 

 the cultivation of plants has its analogue at least in the 

 state of nature. The offspring of a brood differ from one 

 another and from their parents. How many strange 

 sports there are and how unceasing the crop of minor 

 fluctuations. Those w r ho say they see no variation now 

 going on in nature should try a month's work at identi- 

 fying species. 



Morphological.- -There are said to be over half a 

 million species of living animals, about half of them 

 insects. The fact that we can make at least a plausible 

 genealogical tree of animals, arranging them in series 

 along the lines of hypothetical pedigree, is suggestive. 



Throughout long series, structures fundamentally the 

 same appear with varied form and function ; the same 

 bones and muscles are twisted into a variety of shapes. 

 What differences in detail there are between the anterior 

 limb of a frog, the paddle of a turtle, the wing of a bird, 

 the flipper of a whale, the fore -leg of a horse, and the arm 

 of man and yet in all there are the same fundamental 

 bones and muscles. Why this adherence to type if animals 



