THE LAMARCKIAN THEORY 419 



finally became a part of the heritage of the species. Either view 

 has its hypothetical aspects; neither is an adequate explanation 

 of the gradual development which is so clearly shown in evolution- 

 ary series. 



Experimental Evidence. The conditions established for experi- 

 mentation are that a character must appear in response to a 

 definite environmental condition which does not ordinarily occur 

 in the species under consideration, and second, that this character 

 must persist in some degree in later generations produced after 

 the cessation of the environmental influence. 



Experiments dealing with the inheritance of acquired characters 

 have dealt with four kinds of environmental effects, viz., effects 

 upon the soma, effects upon the germ plasm, parallel induction or 

 simultaneous effects upon l)oth soma and germ plasm, and somatic 

 induction or effects upon the soma which are later transmitted to 

 the germ cells. 



While these groups are significant in the classification of experi- 

 mental results, it is obvious that not all of them bear directly 

 upon the problem in hand. The first, effects upon the soma, is 

 obviously of no importance unless some connection with the 

 germplasm can be demonstrated, for we have seen that it is only 

 through the chromosomes of the germ cells that hereditary char- 

 acters are transmitted. Effects upon the germ plasm have been 

 demonstrated in animals treated with X-rays, alcohol, and other 

 agents. Notable among these are Muller's recent results with 

 Di'osophila already cited. It is clear that the germ cells can be 

 modified directly, but characters arising from these modifications 

 are in no sense the acquired characters with which evolution is 

 concerned. Parallel induction is no more than a coml^ination of 

 these two. The only remaining category, somatic induction, ex- 

 presses the condition which nmst occur if significant acquired 

 characters are to be proved inherited. The character must appear 

 in the body of the individual organism and then affect the germ 

 plasm in such a way that it will be reproduced in succeeding 

 generations. 



Weismann's Mice. One of the most famous experiments with 

 somatic modifications was carried out by Weismann. He cut off 

 the tails of young mice generation after generation, breeding the 

 tailless mice and removing the tails which inevitably appeared 

 in their offspring, without affecting in the slightest the develop- 



