TRUE MIMICRY 107 



natural selection is, as has been already shown, nowhere possible: 

 we can never exactly estimate how great the advantage is which 

 a species requiring protection derives from a slight increase in the 

 resemblance to an immune model ; and I for one do not know how we 

 could even definitely prove that a certain species needed a greater 

 degree of protection than it had previously enjoyed in order to ensure 

 its persistence in the struggle. It would be necessary to know the 

 total number of individuals living on a certain area for many genera- 

 tions. If it appeared that there was a progressive diminution in the 

 number of individuals, we should be justified in concluding that the 

 species had not an adequate power of persistence, and that it therefore 

 required a more effective protection. But it is impossible for us to 

 collect such exact data for any species living under natural conditions, 

 although we can often say approximately that a species is progressively 

 decreasing in numbers. Even this, however, we can usually do only 

 in cases which are influenced directly or indirectly by the interference 

 of Man in nature, and in which the falling off in the species occurs so 

 rapidly that there is no time for the slow counteractive influence 

 of natural selection. We shall see later that in this way many species 

 have been eliminated even within historic times. 



I have just spoken of the ' need of protection,' and I have a few 

 remarks to add on that subject. It is a mistake to believe that every 

 ' rare ' species, that is, one represented by few individuals, is already 

 in process of disappearing. It is not the absolute number of individuals 

 that determines the survival of a species, but the fact of the number 

 remaining the same. It is equally mistaken to suppose that an 

 amelioration of the conditions of existence for any species by natural 

 selection is possible only .when its persistence is already threatened ; 

 that is, when the number of individuals (the 'normal number') is 

 steadily decreasing. On the contrary, it is of the essence of natural 

 selection that every favourable variation which crops up is, ceteris 

 paribus, preserved, and becomes the common possession of the species, 

 quite independently of whether this improvement is absolutely 

 necessary to its preservation or not. In the latter case it will simply 

 become a commoner species instead of a rare one ; and every species is, 

 so to speak, striving to become common and widely distributed, since 

 every advantageous variation that can possibly be produced is 

 accumulated and made the common property of the species. But 

 this has its limits, not only in the constitution and the structure 

 of each species, but also in the external conditions of its life. If 

 a species of butterfly be restricted, in the caterpillar stage, to a single, 

 rare species of plant, its normal number will be, and must remain, 



