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 sutficiently 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 action 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. 
