INTRODUCTORY 3 



even the much easier problem, how and by what forces the evolution 

 of the living world has proceeded from a given beginning, is far from 

 being finally settled ; antagonistic views are still in conflict, and there 

 is no arbitrator whose authoritative word can decide which is right. 

 The Hoiv ? of evolution is still doubtful, but not the fact, and this 

 is the secure foundation on which we stand to-day : The world of 

 life, as we know it, has been evolved, and did not originate all at 

 once. 



Were I to try to give, in advance, even an approximate idea of the 

 confidence with which we can take our stand on this foundation, 

 I should be almost embarrassed by the wealth of facts on which 

 I might draw. It is hardly possible nowadays to open a book on the 

 minute or general structural relations, or on the development of any 

 animal whatever, without finding in it evidences in favour of the 

 Evolution theory, that is to say, facts which can only be understood 

 on the assumption of the evolution of the organic world. This, too, 

 without taking into account at all the continually increasing number 

 of facts Palaeontology is bringing to light, placing before our eyes the 

 forms which the Evolution theory postulates as the ancestors of the 

 organic world of to-day : birds with teeth in their bills, reptile-like 

 forms clothed with feathers, and numerous other long-extinct forms 

 of life, which, covered up by the mud of earlier waters, and preserved 

 as f fossils ' in the later sedimentary rocks, tell us plainly how the 

 earlier world of animals and plants was constituted. Later, we shall 

 see that the geographical distribution of plant and animal species of 

 the present day can only be understood in the light of the Evolution 

 theory. But meantime, before we go into details, what may justify 

 my assumption is the fact that the Evolution theory enables us to 

 predict. 



Let us take only a few examples. The skeleton of the wrist in 

 all vertebrate animals above Fishes consists of two rows of small 

 bones, on the outer of which are placed the five bones of the palm, 

 corresponding to the five fingers. The outer row is curved, and there 

 is thus a space between the two rows, which, in Amphibians and 

 Reptiles, is filled by a special small bone. This ' os centrale ' is 

 absent in many Mammals, notably, for instance, in Man, and the 

 space between the two rows is filled up by an enlargement of one 

 of the other bones. Now if Mammals be descended from the lower 

 vertebrates, as the theory of descent assumes, we should expect to 

 find the ' os centrale ' even in Man in young stages, and, after many 

 unsuccessful attempts, Rosenberg has at last been able to demonstrate 

 it at a very early stage of embryonic development. 



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