vi PEEFACE. 



and Phylogeny — variability and heredity ' — is put out 

 of court as useless, for these are merely axiomatic expres- 

 sions for a sum of identical or correlative phenomena of 

 which the essential nature is in no way revealed by them. 

 They all still await an intrinsic explanation. 



It appears to me that of all the properties of the 

 animal organism, Variability is that which may first and 

 most easily be traced by exact investigation to its efficient 

 causes ; and, as it is beyond a doubt the subject around 

 which at the present moment the strife of opinions is 

 most violent, it is that which will be most likely to repay 

 the trouble of closer research. 



I have endeavoured to facilitate this task, so far as in 

 me lies, by here presenting a general view of those facts 

 and hypotheses which bear upon the subject and are either 

 of universal significance or, from my point of view, appear 

 to offer favourable subjects for experimental treatment. 

 This Hst, however, makes no pretension to completeness. 

 If only it should give an impulse to research, on however 

 small a scale, so long as it is systematically conducted and 

 thoroughly carried through — if only it should contribute 

 to extend my own convictions as to the uselessness of 

 casual and disconnected observations, I shall have at- 

 tained my end. 



The immediate occasion of the writing of these papers 

 was a call to deliver a course of twelve lectures at the 

 Lowell Institute at Boston from October to December 

 1877. 



KAKL SEMPEE. 



WiJbzbukg : Sept. 1879. 



