NATURAL SELECTION 231 



everyone that evolution had actually taken place. To that 

 end he endeavoured to collect a large body of evidence that 

 apparently could be explained only on the Natural Selection 

 hypothesis. To-day the much greater body of morphological, 

 taxonomical and embryological evidence is alone almost 

 enough to prove that evolution must have occurred ; and if 

 we admit that living organisms are always derived from pre- 

 vious living organisms, the picture of extinction and gradual 

 change presented by the palaeontological record completes the 

 argument without forcing us to say exactly how evolution 

 happened. In Darwin's day it was legitimate to ask, ' If these 

 structures are not the result of Natural Selection, how do you 

 explain them ? ' To-day we are able to answer, ' We cannot 

 explain them,' and yet not feel that we are betraying science. 

 This digression disposes of the argument that Natural 

 Selection must be all-important because nothing else would 

 explain the facts. There are many things about living organ- 

 isms that are much more difficult to explain than some of their 

 supposed ' adaptations.' 



It is possible to cite a large mass of indirect evidence that 

 has been held to prove that the structural differences that 

 distinguish species and lower categories are related to the lives 

 or behaviour of the animals in question in such a way that they 

 must have arisen on account of their survival value through 

 Natural Selection. We propose to consider part of this matter 

 in detail and part more summarily. A word is, however, 

 necessary beforehand as to our selection and arrangement of 

 the matter. 



Some of the phenomena and observations put forward as 

 evidence for Natural Selection are by now biological classics. 

 The group of observations, etc., on mimicry in divers groups 

 is a standard example of a subject which has been intensively 

 studied over a long period of years. Other cases have had 

 a good measure of attention and experiment given to them, 

 but not on the same large scale as mimicry. Lastly there are 

 a number of isolated instances in which the field of observation 

 is restricted to the differences between a single pair of species. 

 We have arranged our subject-matter under these categories. 



We have not included in this survey a number of miscel- 

 laneous cases of adaptation which are usually explained as due 

 to Natural Selection. There are, for example, the flattening 



