ARE BRITISH BUTtBKFLlES OVER COLLECTED V 205 



the thing, or from any scientific motives. It is so that they can 

 catch those entomologists who have more money than brains, and 

 who want a big series of the rare species. If anyone wants a long 

 series of a scarce insect, let him box a female and rear a series. He 

 will learn twenty times more about the insect than by taking the 

 series of imagines, and ten thousand times more than by buying. 

 Or again, collect a big series over several years. Or if you must buy, 

 buy from a well-known reliable collection, when put up to auction at 

 Stevens', for instance ; but do not buy rare insects from dealers, 

 and more especially from those exterminators who supply the dealers, 

 and anyone else, should they ofier to purchase. If all the societies 

 in the country were to proclaim this to their members, who could in 

 turn tell their friends who do not aspire to belonging to a society, 

 I think we should be on the high road to rectifying a growing evil, 

 which, if not taken in hand, will very soon assume such alarming 

 proportions, that the interest in entomology, as far as these islands 

 are concerned, will be seriously curtailed to all those who collect 

 as a means to an end, and which end is not extinction. 



MIMICRY. 



VI. — Selection guided by utility at work. 

 By J. W. TUTT, F.E.S. 



We have already seen how the variational units of an insect's wing- 

 may differ. The variational unit may consist of a whole wing, of the 

 surface of a wing, of any part of the wing or of a single scale, and 

 we have seen that whilst the variational units result from internal 

 forces, their ultimate course, size and shape, is decided by utility. 



It may be well now to follow Weismann's course of reasoning, 

 through what he conceives to be the niodns operajuH by which a hypo- 

 thetical butterfly reaches a certain pattern of mimetic coloration. 



" Let us suppose," he says, " that the ancestral species of a certain 

 .forest butterfly habitually reposed on branches near the ground, and 

 covered with dry or rotten leaves ; such a species will have assumed 

 on its under surface a protective colouring which by its dark, brown, 

 yellow, or red tints will tend towards a similarity with such leaves. If, 

 however, the descendants of this species should be subsequently com- 

 pelled, no matter from what cause, to adopt the habit of resting higher 

 up, on the green-leaved branches, then, from that period on, the brown 

 colouring would act less protectively than the shades verging towards 

 green ; and a process of selection will have set in which consisted first 

 in giving preference only to such individuals whose brown and yellow 

 tints showed a tendency to green." 



Development in the direction of an increase of green colour, 

 Weismann points out, is only possible on the assumption that the 

 l)iopliors composing the determinants of the scales affected could alter 

 in quantitative proportions, so as to increase the green and decrease 

 the number of brown scales. When the determinants which give rise 

 to green, however, once begin to prevail, "the selective process must 

 continue until the highest degree of faithfulness required by the species 

 in the imitation of fresh leaves has been attained." 



" That the foregoing process has actually taken place is evidenced, 

 not only by the presence of the beginnings of such transformations, as 



