84 EVIDENCES FOR EVOLUTION. 



factor in evolution. On the other hand, if hereditary variations are 

 all indefinite, and natural selection can only act when favourable 

 variations chance to occur, then this factor is all-important in causing 

 evolution. The difficulties in this assumption are — firstly, the enormous 

 time required by the theory of probabilities for the occurrence of 

 favourable variations ; secondly, the inability of natural selection to 

 operate till the variations are sufficiently great to become of some vital 

 importance ; and lastly, the necessary assumption that living matter 

 does not conform to Newton's third law of motion, reactions in the 

 form of variations being produced with no correlation to actioti of the 

 environment. 



Evidences for Evolution. — We may conclude this 

 chapter by mentioning a few of the chief evidences which 

 lead zoologists to believe in the evolution of the organic 

 world : — 



1. The animal kingdom can be arranged in a series, 

 according to structure, which forms a more or less unbroken 

 gradation from lowest to highest. 



2. Certain structures, called vestiges and rudiments, can 

 be best explained as examples of parts of an organism which 

 are either in their earliest or their last stages of evolution. 



3. On an non-evolutional hypothesis the species should 

 and must form the lowest unchangeable unit, and yet it is so 

 variable that it is found quite impossible in any particular 

 case to define a species. 



4. Series of fossil forms have in certain instances, e.g., 

 the crocodile and horse, enabled scientists to actually re- 

 create all the stages in the evolution of the group. 



5. Facts of geographical distribution, such as the fauna 

 of oceanic islands and discontinuous distribution, are un- 

 explainable by any other hypothesis. 



