INHERITANCE OF ACQUIRED CHARACTERS 51 



the light of the most recently discovered principles of heredity. A very 

 important conception in this connection is the continuity of the germ 

 plasm, another is the variability of the potency of unit characters. 



Admitting, then, that certain acquired characters have actually 

 appeared in later generations, we should consider, first, whether or not 

 the germ plasm has been changed by the stimulus which has produced 

 the changes in the body. It has been shown that starvation in the larval 

 stages of insects will produce dwarfs in later generations, but here it is 

 assumed that the unfavorable conditions surrounding the germ plasm 

 persist and that there is no real change in the composition of the germ 

 plasm. Can Kammerer's results be explained in the same way? Of 

 course a Lamarckian can not be asked to produce a form which will 

 not revert. The only test that can be readily applied is that of Men- 

 delian inheritance. It has been shown by the author that in one case 

 at least the new factor behaved like a Mendelian factor. Tower also 

 found this true in crossing a pale potato beetle, which he derived ex- 

 perimentally, with a beetle of the normal color. Such a test to discover 

 a change in the composition of the germ plasm is certainly very 

 significant. 



Granted, then, that the germ plasm has been changed, we should 

 next consider whether it has been changed directly or indirectly. The 

 experiment of keeping tadpoles in water for an abnormally long time 

 showed that in order to affect the next generation the stimulus must 

 continue to act until the sex cells are mature. Tower also came to the 

 same conclusion in his experiments on the potato beetle where heat was 

 the stimulus. The changes, then, are probably due to the direct action 

 of chemical and physical stimuli on the germ plasm contained in the 

 ripe germ cells, exactly as MacDougal produces mutations, as he claims, 

 by injecting chemicals into the ovary of a plant. But why should the 

 stimuli not effect the germ plasm of the embryo as well, since, accord- 

 ing to the theory of continuity, the same plasm is always present even 

 in the youngest stages? It may possibly be claimed that, if any such 

 effect is produced in the embryo, the change is repaired before repro- 

 duction takes place. 



Granted, then, that the germ plasm in these cases is more or less 

 directly affected by the environment, we should consider whether the 

 change is more than a change of potency of a factor already present. 

 According to Castle such potency may be increased by selection. Per- 

 haps the new environment may increase in some way the potency of a 

 factor which is present in a weak condition. For example, in the case 

 of the spotted salamander, the potency of the factor represented by the 

 yellow pigment may possibly be changed by the action of the yellow 

 light, which actually increases the amount of the pigment in the body 

 of the adult until perhaps the nature of the fluids of the body cavity 



