INTRODUCTORY 27 



and that the precision with which a species conforms to its 

 pattern is an indication of the closeness of that control. Anyone 

 familiar with the characteristics of Moths will agree that the 

 Noctuids, Geometrids and Tortricids are creatures whose existence 

 depends in some degree on the success with which they can escape 

 detection by their enemies in the imaginal state. We are there- 

 fore not surprised to find that some species of these orders 

 exhibit definite geographical variation in conformity with the 

 character of the ground, which may reasonably be supposed to 

 aid in their protection. If this were all, there would be nothing to 

 cause surprise. We might even be disposed to allow that varia- 

 bility might contribute to the perpetuation of animals so situ- 

 ated, on the principle that among a variety of surroundings 

 some would probably be in harmony with the objects on which 

 they rest. But we cannot admit the plausibility of an argument 

 which demands on the one hand that the extreme precision with 

 which species A adheres in the minutest details of its colour and 

 pattern to a certain type shall be ascribed to the protective fitness 

 of those details, and on the other hand that the abundant varia- 

 bility of species B shall be ascribed to the same determination. 

 If it is absolutely necessary for A to conform to one type how 

 comes it that B may range through some twenty distinct forms, 

 any two of which differ more from each other than the regular 

 species of many other genera? The only reply I can conceive 

 is a suggestion that there may be some circumstance which 

 differentiates the various classes of cases, that the exigencies 

 of the fixed species may be different from those of the variable. 

 Those who make such appeals to ignorance do not always perhaps 

 realise whither this course of reasoning may lead. If admissible 

 here the same argument would lead us to suggest that because 

 albino moles have for an indefinite period occurred on a certain 

 land near Bath there may be something in the soil or in the 

 conditions of life near Bath which requires a proportion of albinos 

 in its mole population. Or again, because the butterfly Thais 

 rumina in one locality, Digne in the south of France, has a per- 

 centage of individuals of the variety Honor atii (with certain 

 normally yellow spots on the hind wing coloured bright red) 



