356 PRINCIPLES OF ANIMAL BIOLOGY 



he unprofitable. If these paragraphs have shown what the problem of 

 the cause of evolution is, and what sort of solution of it is to be expected, 

 they have served their purpose. 



Changes in the Environment. — Although environment is not gener- 

 ally accepted without reservations by leading biologists as an important 

 cause of evolution, yet from its historical interest and from the behef 

 still held by many biologists, it should be mentioned. Animals are often 

 so obviously fitted for the environment in which they live, that observers 

 are apt to conclude that in some way the environment worked them over 

 until they were fit. Though it is not improbable that adaptation to 

 environment is due to the migration of animals to that environment to 

 which they are best fitted, provided they have had time to reach it and 

 there is nothing in the way, environment as a cause of evolution cannot 

 thus be swept aside without consideration. There are, as stated above, 

 many biologists who firmly beUeve in the environment as an agent in 

 bringing about evolution. That was Lamarck's view a century ago, and 

 the same view is widely held today. As pointed out in the first chapter, 

 some paleontologists, many medical men, and most of the laity are 

 adherents, even now, of the Lamarckian theory. In support of their 

 position may be cited the work of a few biologists who believe they have 

 Ijeen able, in experiments, to produce by artificial means, such as changes 

 of the medium, temperature, nutrition, or moisture, modifications which 

 were subsequently inherited. In so far as these experiments have dealt 

 with bacteria and other simple organisms, or the germ cells have been 

 directhj affected by the environment, the conclusions are in part apparently 

 justified; but when they involved higher animals, and when the germ 

 cells were not directly influenced, the experiments have never satisfied 

 the majority of experimental biologists. Nevertheless it would be unfair 

 to disregard these experiments. There is no a priori ground for supposing 

 that environment could not effect evolution, and it may be that some 

 day evidence so conclusive will be brought forth that everyone will 

 admit its vahdity. However, that evidence, if it is ever secured, may 

 easily prove unsatisfactory to those persons who most dihgently seek it. 

 For the strong appeal to the imagination made by the theory of environ- 

 mental agency in evolution hes in the possibility that adaptation to 

 environment may thereby be explained. It is well known that changes 

 in the environment cause alterations in the bodies of animals and plants, 

 and that some of these modifications are advantageous to the organisms. 

 But in no case has it been estabhshed that one of these adaptive altera- 

 tions has been transmitted to the descendants. Indeed, it is doubtful 

 whether there exists in organisms any mechanism by which a purely 

 somatic change could produce an effect on the germ-plasm such that the 

 same somatic character would appear in the next generation. However, 

 the question whether the environment can produce permanent adaptive 



