206 TRANSACTIONS OP SECTION D. 



form may serve as a stepping-stone for a likeness to another ; or, again, the 

 existence of clusters, as they may be called, of forms varying Ln affinity, but 

 embodying a transition by easy stages from one extreme to another. In a case 

 of this sort the objection that may be felt as to two terms in the series arbi- 

 trarily or accidentally picked out is seen to be groundless when the whole 

 assemblage is taken together. 



Much attention has lately been given to the fact that of individual variations 

 some are transmissible by heredity and some are not ; under the latter heading 

 would generally fall somatic modifications directly induced upon the individual 

 by conditions of environment. Whether any other kind of variation belongs 

 to the same category need not for the present purpose come into discussion. 

 But with regard to the undoubtedly transmissible variations, or mutations if 

 we like to call them so, there is, I think, a fairly general consensus of opinion 

 that they need not necessarily be large in amount. A complete gradation in 

 fact appears to exist between a departure from type so slight as to be scarcely 

 noticeable and one so striking as to rank as a sport or a monstrosity. And we 

 know now that where the Mendelian relation exists between two forms, no 

 amount of interbreeding will abolish either type; intermediates, if formed, are 

 not permanent, and if one type is to prevail over the other it must be by means 

 of selection, either natural or artiiicial. 



In view of all these considerations I venture to think that there is no reason 

 to dispute the influence of natural selection in the production of these remarkable 

 resemblances. Other interpretations may no doubt be given, but they involve 

 the ignoring of some one or more of the facts. It may fairly be claimed that 

 the theories of Wallace, Bates and Miiller, depending as they do on a basis of 

 both observation and experiment, come nearer to accounting for the facts than 

 any other explanation as yet offered. It will of course always be possible to 

 deny that any explanation is attainable, or to assert that we ought to be satisfied 

 with the facts as we find them without attempting to unravel their causes. But 

 such an attitude of mind is not scientific, and if carried into other matters 

 would tend to deprive the study of Nature of what to most of us is its princiual 

 charm. It is quite true that before the validity of any generalisation is 

 accepted as finally and absolutely established, every opportunity should be taken 

 of deductive verification. This has been fully recognised by the supporters of 

 the theory of mimicry, and much has been done to test in this manner the 

 various conclusions on which the theory rests. The verification is not complete, 

 and pei'haps never will be, but every successive step increases the probability of 

 its truth ; and probability, as Bishop Butler taught, is the guide of life. 

 Meantime it is, one may say, the positive duty of everyone who has the oppor- 

 tunity, to fill up, so far as is in his power, the gaps that still exist in the chain 

 of evidence. Here is an especially promising field for naturalists resident in 

 tropical regions. 



Before concluding this address, there are two points on which I should like 

 to lay some special emphasis. One is the undesirability — I had almost said 

 folly — of undervaluing any source of information, or any particular department 

 of study, which does not come within the personal purview of the critic or 

 commentator. 'I hold,' says Quiller-Couch, 'there is no surer sign of intel- 

 lectual ill-breeding than to speak, even to feel, slightingly of any knowledge 

 oneself does not happen to possess.' This is a temptation to which many of us 

 are liable ; and falls, I fear, are frequent. It was a matter of sincere regret 

 to me to find one of my most valued scientific friends speaking publicly of the 

 Odes of Horace as a subject comparatively devoid of interest. I can only 

 confess my utter inability to sympathise with my friend's point of view. If he 

 had merely said, ' excellent as those works may be, I have other things to do 

 than to attend to them,' I could approve; but that is a different matter. The 

 failing that I speak of is unfortunately by no means unknown among scientific 

 men, and is perhaps rather specially prevalent when such subjects as those of 

 my present address are in question. I can recall a very eminent man of science, 

 no longer living, speaking with scarcely veiled scorn of those who occupied 

 themselves with ' butterflies in cases.' This was in a Presidential Address to 

 a Section of this Association. If so little respect is paid by a leader of science 

 to work done in another part of the field, it is perhaps not to be wondered at 



