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I have long maintained, and have previously suggested in 

 this paper, that many of the differences in species are 

 essentially physiological, and depend upon differences of 

 constitution, performance of functional and vital processes, 

 habit, &c, and that the morphological characters of the 

 species are often only an outward expression of such 

 differences. It is well known that certain species of Lepi- 

 doptera are restricted to a single species of food-plant, and 

 will starve if it be not forthcoming, whilst many of its allies 

 may be more or less general feeders. Here evidently we 

 have to deal with a species that is probably maintained dis- 

 tinct purely from this physiological peculiarity, which will 

 sufficiently isolate it from all its congeners. As a principle 

 this is pretty generally held to be true by the more advanced 

 Darwinians, and as far back as 1883 Meldola wrote, " It is 

 most important to bear in mind that Darwin's prime mover, 

 natural selection, acts not only upon external characters, 

 but likewise upon internal organisation ; minute constitutional 

 or physiological deviations, at present utterly beyond the 

 ken of science, can be seized upon and perpetuated by this 

 agency when of any advantage to the possessor. The 

 survival of the fittest is utilitarianism in excelsis" Later, in 

 1897, the same writer observes, " If variability of nervous 

 function can be seized upon by natural selection, it is but 

 reasonable to suggest that variability of other internal func- 

 tions can also be utilised when of advantage. Of the 

 functions discharged by the internal organs other than the 

 nervous system, all those obscure chemical processes con- 

 cerned with metabolism and nutrition, waste and repair, 

 secretion and excretion, and so forth, must be in adjustment 

 with the life conditions of the organism. If species are 

 adapted to their mode of existence, as is admitted by all 

 schools of evolutionists, the selectionist must explain the 

 physiological adaptation in the same way that he explains 

 the structural adaptation, viz. by the survival of individuals 

 whose physiological processes are best in harmony with 

 their mode of life ; but this explanation starts from a 

 variability of physiological function. From analogy with 

 the known variability of structure it is fair to infer that a 

 physiological variability also exists, of the range of which 

 we are at present ignorant. At any rate, it appears to me 

 inconceivable that any change of environment requiring a 

 modification of structure of sufficient magnitude to rank as 

 diagnostic in the systematic sense should not also be 

 accompanied by a greater or less amount of physiological 



