1898] THE STUDY OF VARIATIONS 233 



to the direct action, we must bear in mind, as Professor Weis- 

 mann has lately insisted, and as I have incidentally shown in 

 my work on ' Variation under Domestication,' there are two 

 factors — namely, the nature of the organism and the nature of 

 the conditions. The former seems to be much the more im- 

 portant ; for nearly similar variations sometimes arise under, 

 as far as we can judge, dissimilar conditions ; and, on the 

 other hand, dissimilar variations arise under conditions which 

 appear to be nearly uniform. The effects on the offspring are 

 either definite or indefinite. They may be considered as 

 definite when all or nearly all the offspring of individuals, 

 exposed to certain conditions during several generations, are 

 modified in the same manner " (p. 6 " Origin of Species," 6th 

 edition, 1884). 



" Indefinite variability is a much more common result of 

 changed conditions than definite variability, and has probably 

 played a more important part in the formation of our domestic 

 races " (p. 6 " Origin of Species "). 

 Weismann. — " The cause of hereditary variation must be due to the 

 direct effect of external influences on the biophors and deter- 

 minants " (p. 415, Weismann, "Germ Plasm: A Theory of 

 Heredity"). 



" We can none the less avoid assuming that the elements of 

 the germ-plasm — i.e., the biophors and determinants — are 

 subject to continual changes of composition during 

 their almost uninterrupted growth, and that these very 

 minute fluctuations which are imperceptible to us 

 are the primary cause of the greater deviations in 

 the determinants, which we finally observe in the 

 form of individual variations" (p. 417, " Germ Plasm "). 



" Of course, I see no reason for assuming two kinds of 

 hereditary variations different in origin. Still, it is likely 

 that only a relatively small proportion of the numberless in- 

 dividual variations lie on the path of phyletic advancement, 

 and so under the guidance of germinal selection mark out 

 the way of further development ; and hence it would be quite 

 possible in this sense to distinguish definitely directed 

 individual variations from such as fluctuate hither and thither 

 with no uniformity in the course of generations. The root of 

 two is, of course, the same, and they admit of being distin- 

 guished from each other only by their success, phyletic modi- 

 fication, or by their failure" (Note, p. 17, "Germinal Selection"). 

 Henslow. — " I would describe the process, therefore, once more as 

 the result of the responsive power of protoplasm, on the one 

 hand, and the forces of the external environment on the other. 



B 



