RELATION TO SEXUAL DTFFEHENTIATION. 49 



development. Even evolutionary changes appear to depend largely 

 upon the power of alternative expression. After a character has once 

 been acquired transmission seems to be permanent. Characters that 

 are discarded from expression are not dropped from transmission, but 

 may be transmitted in latent or rudimentary form for thousands of 

 generations, as the facts of recapitulation and reversion have shown. 

 The transmission of latent characters shoukl not be considered as a rare 

 or exceptional phenomenon, but as the normal, universal condition. 



The internal agencies of the cells, that determine the expression of 

 characters, remain active and capable of profound readjustments 

 during the life history of each individual plant. The changes of 

 characters shown in mutative variations are considered as very im- 

 portant phenomena of heredity, and yet they are far exceeded by the 

 changes that regularly take place during the development of every 

 normal plant. Even the metamorphoses of insects hardly constitute 

 such profound modifications of form and structure as the differences 

 among the internode members of the same plant. 



Though the facts of plant development seem to afford little ground 

 for the application of Weismann's idea of a fundamental distinction 

 between the germ plasm and the somatic tissues, a distinction is at 

 least to be made between the processes of inheritance in plants and 

 animals. The unknown internal mechanism that controls the ex- 

 pression of the characters evidently remains in a much more active 

 state during the development of a plant than in the case of an animal. 

 This consideration may help to explain the generally recognized fact 

 that the characters of plants are much more readily modified by 

 changes of environment than those of animals. A recent writer has 

 proposed to explain the greater adaptability of plants and lower 

 animals to changes of environment by framing general laws of dimin- 

 ishing environmental influences in passing from lower to higher 

 groups.^ 



A study of the methods of reproduction and development followed 

 in the various groups may reveal biological facts underlying tliis 

 generalization. The higher animals, that show the least susceptibility 

 to environmental modification, not only have a more nearly simul- 

 taneous determination of the expression of the characters, but their 

 warm-blooded bodies are able to maintain constant temperatures 

 and thus protect themselves against the fluctuations of heat and cold 

 that represent one of the most disturbing factors in the development 

 of plants. 



Consideration should also be given to the possibility that the sudden 

 and complete changes of characters involved in the production of the 



1 Woods, F, A. Laws of Diminishing Environmental Influences. Popular Science Montlily, April, 

 1910, p. 313. 



221 



