5o6 NATURAL SCIENCE. s„,,. 



so in the past undue importance has been attached to the part I have 

 supposed them to play. To guard against this misunderstanding as far 

 as possible, I will refer to a case in which it must be obvious that I do 

 not suppose that the particular influence I shall refer to has anything 

 to do with heredity in any accepted sense of the word. I regard the 

 likeness of one individual to another as arising from a likeness in the 

 causes which determined the characters of the two individuals respec- 

 tively. Into those causes some external influences have entered as 

 factors, direct or indirect, but the external influences dwindle into 

 insignificance when compared with the internal ones. 



Professor Weismann records an experiment of his, in which he 

 cut off the tails of many very young mice of half-a-dozen successive 

 generations. All those mice, when fully developed, were tailless 

 mice. They resembled each other in being mice by virtue of the 

 action of internal influences, which were approximately alike in all 

 cases. They resembled each other in being tailless by virtue of the 

 action of one particular external influence, which was alike in all 

 cases. So long as that external influence remained constant, tailless- 

 ness of the full-grown mice was a constant character, and no longer. 

 When the influence to which that taillessness was due was removed, 

 the taillessness ceased to recur. 



If some change in the conditions of existence should so alter the 

 constitution of mice that the fertilised eggs they produce should have 

 such structure as would lead to their development into tailless mice, 

 then there is reason in believing that the maintenance of the changed 

 conditions would convert the mouse into a tailless species, and that 

 the taillessness of the species would remain a constant character so 

 long as no further change in the conditions of existence occurred, and 

 no longer. Though the specific characters are determined by the 

 joint operation of internal and external influences, the internal 

 influences are infinitely more effective than the external, as they are 

 also infinitely more complex. But, on the other hand, the internal 

 influences depend botli directly and indirectly upon the external ones. 

 And, further, none but slight changes in the external influences are 

 compatible with the continued existence of the species. 



The direct, though perhaps remote, influence of external 

 influences upon the internal constitution, and hence more remotely 

 upon the constitution of the ova and spermatozoa, and hence again 

 more remotely still upon the form and structure of succeeding 

 generations, is well seen in the transformation of a stock of Artemia 

 salina into A. muelhauseni, and vice versa by Schmankewitsch through 

 alteration of the strength of the salt-water in which the animals 

 were living. This result is so interesting that it is surprising that 

 nobody with the means at command has yet been found to verify it by 

 further experiments. The transformation of a stock of Artemia 

 into Bvanchipus is so astounding, that though it might strengthen my 

 argument I dare not assume it to be true till it has been verified. 



