354 PRINCIPLES OF ANIMAL BIOLOGY 



separated from the other continents (Fig. 244). No ungulates are native 

 to Australia. If ungulates came into existence in any other way than 

 by evolution from a common stock, there is no apparent reason why they 

 should not have arisen in Australia. But if evolution be assumed as the 

 only method of originating new forms, it is not difficult to understand 

 why Austraha never received representatives of this particular group. 

 The ancestors lived elsewhere, and the migration of the descendants to 

 Australia was barred by the sinking of the land. 



The facts stated in the foregoing paragraphs, gathered from the 

 provinces of comparative anatomy, embryology and physiology, from 

 paleontology and distribution, receive a very simple and plausible ex- 

 planation if evolution is assumed. These facts are only a few among a 

 host that would have served equally well. Although these facts have not 

 been specifically designated as evidence of evolution, they constitute 

 such evidence. Facts which may be satisfactorily explained by a theory, 

 and are not readily explained in any other way, are evidence in favor of 

 that theory. To distinguish such evidence from that which comes from 

 direct observation, it may be termed evidence by inference. The time 

 was when inferential evidence was the chief, if not the only, kind advanced 

 to support evolution. The doctrine of evolution was firmly established 

 by evidence from paleontology, comparative anatomy, comparative 

 embryology, and so on; for, at the time of the rise of the evolution theory, 

 new characters had seldom been observed to arise among carefully 

 pedigreed animals. Pedigrees were not common in the experiments of 

 scientific men, and those kept by practical breeders were not always 

 accurate in detail. Ver}' few new characteristics had ever been observed 

 at the time of their origin under circumstances which forbade their 

 being attributed to hybridization or to the environment, until after the 

 evolution theory had been generally accepted by biologists. The 

 evidence which, as stated above, seems to us the best evidence, namely, 

 the observation of mutations at the time of their origin, was not dis- 

 covered in quantity until near the end of the last century, and since that 

 time. 



CAUSES OF EVOLUTION 



While no thinking person now denies the fact of evolution, there is 

 still much disagreement regarding the causes or method of the process. 

 There are not wanting facts which bear on the question, but it is probable 

 that a good many more facts are undiscovered or even undiscoverable. 

 The question as to the causes of evolution must therefore be answered, 

 if answered at all, in a rather speculative fashion. Before entering 

 upon such speculation, it is essential to })e perfectly clear as to what the 

 problem is. As was pointed out early in the chapter, evolution occurs 

 whenever a new character arises which is capable of being inherited. 



