112 CHAPTERS ON EVOLUTION. 



for grasping the branches of trees those of the ground-dwelling 

 insect or root-eating bandicoots and those of some other Australian 

 marsupials (Fig. 49, B, c) should all be constructed on the same extra- 

 ordinary type, namely, with the bones of the second and third digits 

 extremely slender and enveloped within the same skin, so that they 

 appear like a single toe furnished with two claws. Notwithstanding 

 this similarity of pattern, it is obvious that the hind feet of these 

 several animals are used for as widely different purposes as it is 

 possible to conceive. The case is rendered all the more striking by 

 the American opossums, which follow nearly the same habits of life 

 as some of their Australian relatives, having feet constructed on the 

 ordinary plan. Professor Flower, from whom these statements are 

 taken, remarks in conclusion : ' We may call this conformity to type, 

 without getting much nearer to an explanation of the phenomenon ; ' 

 and he then adds, 'But is it not powerfully suggestive oTtrue relation- 

 ship, of inheritance from a common ancestor ? ' " To say that things 

 were simply created so after a creative plan, may be a confession of 

 faith ; it is in no sense a scientific explanation with which the mind 

 may grapple so as to arrive at its true significance. 



But the theory of descent goes still further. It also supplies an 

 answer to the obvious question which awaits the naturalist, " How are 

 the variations seen in the limbs of vertebrates to be accounted for ? " 

 Admit that the varied limbs of vertebrates are but so many modifica- 

 tions of a common type, and that, as such, they were derived from 

 their ancestors, to what process do they owe their subsequent 

 modification to the varied wants and ways of life of their predecessors ? 

 The agreement in fundamental structure, as we have seen, is the 

 result of inheritance ; to what law of life do we owe the variations in 

 function the limbs exhibit? The answer to this query lies in a 

 single word adaptation ; that is, the modification of the primitive 

 type of limb for the special circumstances of each animal's life. The 

 essential principle and strength of evolution, consists in its ability to 

 show, firstly, that alteration and modification of an animal's structure 

 take place according to its requirements, and as determined by the sur- 

 roundings of its life; and secondly, that such variations as are favour- 

 able or profitable will be preserved. Such modifications as would fit 

 a limb for swimming or for flight might take place without any violent 

 or sweeping alteration of the limb-type as a whole. We know as a fact, 

 that the skeletons of some domesticated and artificially bred animals, 

 such as the pigeons, are liable to alteration and modification of structure 

 without change of the type of bony framework ; and so with the limbs, 

 which, as mere appendages, are infinitely more susceptible of alteration 

 and adaptation to new ways of life. Thus is illustrated the principle of 

 " natural selection," which constitutes the key-note of Darwinism, and 

 which contends for the preservation of those variations and alterations 



