THE INFLUENCES OF ENVIRONMENT 267 



germ-plasm, and which would therefore bring about variations, either 

 in all or only in certain determinants. In this case all the individuals 

 would vary in the same way, because all would be similarly affected 

 by the same causes of change. 



This is actually the case ; it is indubitable that external influences, 

 such as those emanating from the environment or media in which 

 species live, are able to cause direct variation of the germ-plasm, 

 that is, permanent, because hereditary variations. We have already 

 referred to this process and called it ' induced germinal selection.' 



That such influences of environment may bring about changes 

 in individucd organisms is obvious enough ; that, for instance, good 

 nutrition makes the body strong and vigorous, that too abundant 

 food makes it fat and causes degeneration, that insuSicient food lessens 

 its stamina and vigour, are well-linown facts. We have to inquire, 

 on the one hand, to what extent such influences are able to cause 

 changes in the individual body in the course of a lifetime, and, on 

 the other hand, more particularly, how far such changes or modifica- 

 tions of the soma can call forth corresponding variations in the 

 determinant system of the germ-cells, and whether and under what 

 circumstances they may be transmitted ; for where this is not the 

 case there can be no permanent hei-editary variation of the whole 

 species, and the variation will only persist as long as the conditions 

 which gave rise to it endure, and will disappear again with these. 



The influence of nutrition as a cause of variation has often been 

 over-estimated. The old statement which has gone the round of the 

 textbooks since the time of John Hunter, that the stomach of 

 carnivoi'es may be transformed by vegetable diet into a herbivore 

 stomach, is absolutely unproved. Brandes at least, who not only 

 subjected all the statements in the literature on this point to a critical 

 investigation, but also instituted experiments of his own, regards the 

 statement as altogether unfounded. All the ' cases ' cited, in which 

 the stomach of a gull or of an owl fed on grain became transformed 

 into an organ with stronger muscles and covered with horny plates, 

 depend, according to Brandes, upon inexact observation. There can 

 therefore be no question of any inheritance of this fictitious stomach- 

 transformation, and the idea that such a fundamental histological 

 adaptation as the alleged transformation of the stomach of the grain- 

 eating bird should arise as a direct effect of the food is wholly with- 

 out foundation. 



But it is quite otherwise with purely quantitative differences in 

 nutrition. That meagre diet influences individuals unfavourably is 

 indubitable, and we are certainly justified in considering whether 



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