406 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



fancy that physical disease is but one of the "errors of mortal 

 mind." 



Now, it is undoubtedly true that many adaptations, to cite Prof. 

 Bateson once more, are " not in practice a very close fit." Even the 

 eye, as Helmholtz long ago taught us, has some defects as an optical 

 instrument ; nevertheless, it enables us to see well enough to discern 

 some food for reflection concerning adaptations among living things. 

 And it is my impression that efforts to explain adaptations are likely 

 to continue for the reason that naturalists as a body, perhaps in- 

 fluenced by Huxley's definition of science, have an obstinate habit 

 of clinging to their common sense. 



At the present day there is no longer the smallest doubt of the 

 great outstanding fact that few complex structural adaptations — 

 it would probably be correct to say none such — have come into 

 existence at a single stroke; they have moved forward step by step 

 to the attainment of their full degree of perfection. What has 

 dominated the direction and final outcome of such advancing lines? 

 We can not yet answer this question with an}' degi'ee of assurance; 

 but. procrastinate as we may, it must in the end squarely be faced. 

 We have seen one theory after another forced back within narrower 

 lines or crumbling away before the adverse fire of criticism. I will 

 not pause to recount the heavy losses that must be placed to the ac- 

 count of sexu.al selection, of neo-Lamarckism, of orthogenesis. Some 

 naturalists no doubt would assign a prominent place in this list of 

 casualties to natural selection, but probably there are none who 

 would hold that it has been destroyed utterly. The crux lies in the 

 degree of its efficacy. Stated as an irreducible minimum, the sur- 

 vival of the fit is an evident fact. Individuals that are unfitted to 

 live or to reproduce leave few or no descendants — so much, at least, 

 must be admitted by all. But does this colorless and trite conclusion 

 end the matter or adequately place before us the significance of the 

 facts? Just here lies the whole issue. Does destruction of the unfit 

 accomplish no other result than to maintain the status quo, or has it 

 conditioned the direction of progress? Accepting the second of these 

 alternatives, Darwin went so far as to assign to it a leading role 

 among the conditions to which the living world owes its existing 

 configuration. Since his time the aspect of the problem has widely 

 changed. We must rule out of the question the origin of neutral or 

 useless traits. We must not confuse the evolution of adaptations with 

 the origin of species. We must bear in mind the fact that Darwin 

 often failed to distinguish between non-heritable fluctuations and 

 hereditary mutations of small degree. We are now aware that many 

 apparently new variations may be no more than recombination prod- 

 ucts of preexisting elements. We should no doubt make a larger 



