MODERN PHILOSOPHICAL BIOLOGY. 601 



doctrine wliich was supposed to have been disproved tAvo hundred 

 years since, and which reappeared in the Last century only to be as- 

 sailed with Voltaire's sarcasms. I mean the theory of spontaneous 

 generation, so called a self-contradictory phrase, by which it w^as 

 intended to assert that organisms are produced out-and-out without 

 tlie aid of parents resembling them. While admitting that genera- 

 tion, sexual or asexual, is the mode of reproduction found among ani- 

 mals possessed of complex structure, the partisans of spontaneoiis 

 generation held, on the strength of their exjoeriments, that certain 

 very low organisms might be developed spontaneously, without spe- 

 cific germs, in infusions of organic substances. But though in this 

 dispute experiment has given no definitive verdict nor, indeed, w'as 

 such verdict to be expected still, all the probabilities are on the side 

 of those who assert the universality of generation by means of germs 

 developed in the parents; and, in the absence of experimental demon- 

 stration, we are not without theoretic arguments against the spon- 

 taneous generation of the comparatively high organisms developed in 

 infusions. If this doctrine is to be retained, it is not for the purpose 

 of explaining the formation of organisms, a thing well enough ex- 

 plained without it, but in order to account for the production of really 

 primitive living things i. e., for the appearance of life in a fraction 

 of organic substance, whether this is still possible in our day, or 

 whether it was possible only at a time when, under conditions un- 

 known to us, organic substance originated upon the earth. Thus 

 stated, the question does not depend on experimentation ; it becomes 

 a mere exercise of the imagination, and the result is valueless. 



"Whatever is to be thought of the theory of the beginnings of life, 

 one or more first living beings having appeared upon the earth, after 

 the latter had become capable of supporting them, the question arises 

 as to the transition from the primitive simplicity to the enormous 

 degree of variety now existing. Here we have the problem of the 

 oiigin of species, which is solved by the theory of descent, sometimes 

 denominated transformism. The old conception ot living Nature as 

 an infinitely vai'ied assemblage of organisms which faithfully copy cer- 

 tain types, all of whose parts are governed by the law of final causes, 

 in our time gives way, not without a fierce struggle, before a new 

 conception, which represents living Nature as an infinitely varied as- 

 semblage of organisms which are ever varying under the influence of 

 external circumstances, while under the influence of heredity they 

 tend to fix in a type the results of previous variations. At one time 

 we have, as in breeding, artificial selection; at another time, as among 

 people who have not yet discovered the laws of breeding, a selection 

 that, though unsystematic, is nonetheless real ; finally in Nature, with- 

 out human intervention, a selection based simply on the conditions of 

 existence. In natural selection, the action of which is by far the most 

 general and powerful, the fixing of variations results from adaptation 



