1890 PHYSIOLOGICAL SELECTION 163 



paper was the outcome of many years' study of the 

 philosophy of evolution, during which time he had 

 gradually been coming to the conclusion that natural 

 selection cannot be regarded as the sole guiding factor 

 in the production of species, but that there must be 

 some other cause at work in directing the course of 

 evolution. 



The theory of natural selection rests on two 

 classes of observable facts : first, that all plants and 

 animals are engaged in a perpetual struggle for ex- 

 istence, there being in every generation of every 

 species a great many more individuals born than can 

 possibly survive ; and secondly, that the offsprings, al- 

 though closely resembling the parent form, do present 

 individual variations. It follows, therefore, that those 

 individuals presenting variation in any way beneficial 

 to them in the struggle for existence will survive as 

 being the fittest to do so. Nature, so to speak, selecting 

 certain individuals of each generation, enabling them 

 not only to live themselves, but also to transmit their 

 favourable qualities to their offspring. If a special 

 hne of variation is in some way preserved, there may 

 result a variety so fixed and so distinct from the 

 parent and collateral related forms as to constitute a 

 separate species. 



Further, since the environment {i.e. the sum total 

 of the external conditions of life) is continually 

 changing, it follows that natural selection may slowly 

 alter a type in adaptation to the slowly changing 

 environment, and if in any case the alterations 

 effected are sufficient in amount to lead naturalists to 

 name the result a distinct species, then natural selec- 

 tion has transmuted one specific type into another. 



Mr. Romanes pointed out that the theory of 

 natural selection only accounts for such organic 

 " changes as are of use to the species — by use signify- 

 ing life-preserving — that it is, in fact, a theory of the 



M 2 



