SPONTANEOUS GENERATION AND EVOLUTION : CONCLUSION 391 



of life, and not merely those which have actually been evolved. If 

 this were not so, Man could not still call forth new animal and plant 

 forms, as he is continually doing among our domesticated animals and 

 cultivated plants, just as the chemist is continually 'creating' new 

 combinations in the laboratory which have probably never yet 

 occurred or been formed on the earth. But just as the chemist does 

 not really ' create ' these combinations, but only brings the necessary 

 elements and their forces together in such a combination that they 

 must unite to form the desired new body, so the breeder only guides 

 the variational tendencies contained in the germ-plasm, and con- 

 sciously combines them to procure a new race. And what the breeder 

 does w^ithin the narrow limits of human power is being accomplished 

 in free nature, through the conditions which allow onl}' what is lit 

 to survive and reproduce, and thus bring about the wonderful result 

 — as though it were guided by a superior intelligence — the adaptation 

 of species to their environment. 



Thus in our time the great riddle has been solved — the riddle 

 of the origin of what is suited to its purpose, without the co-operation 

 of purposive forces. Although we cannot demonstrate and follow out 

 the particular processes of transformation and adaptation in all their 

 phases with mathematical certainty, we can understand the principle, 

 and we see the factors through the co-operation of which the result 

 must be brought about. It has lately become the fashion, at least 

 among the younger school of biologists, to attach small value to natural 

 selection, if not, indeed, to regard it as a superseded formula ; 

 mathematical proofs are demanded or, at any rate, desired. I do not 

 lielieve that we shall ever arrive at giving such proofs, but we shall 

 undoubtedly succeed in clearing up miich that nov/ remains obscure, 

 and in essentially modifying and correcting manj^ of the theories 

 we have formed in regard to this question. But what has been 

 already gained must certainly be regarded as an enormous advance 

 on the knowledge of fifty years ago. We now hnmv that the modern 

 world of organisms has been evolved, and we can form an idea, though 

 still only an imperfect one, how and through the co-operation of what 

 factors it could and must have evolved. 



When I say '}nu8t, this refers only to the course of evolution from 

 a given beginning ; but as to this beginning itself, the spontaneous 

 generation of the lowest Biophorids from inorganic material, we are far 

 from having understood it as a necessary outcome of its causes. And 

 if we have assumed it as a reasonable postulate, we by no means seek 

 to conceal that this assumption is far from implying an understanding 

 of what the process of biogenesis was. I do not merely mean that 



