EXCESSIVE VARIATION 55 



the adaptation has become ever more perfect — that is to 

 say, in ever-increasing harmony with the environmental con- 

 ditions. 



There being only two possible kinds of variation among the 

 determinants of the germ-plasm — namely, plus or minus varia- 

 tions — it is obvious that the chances of a variation in one or 

 other of these directions are far greater than the chances that 

 the plus or minus determinants of a. given organism simply 

 balance one another as a result of reduction and amphimixis. 

 This dominant tendency to variation possessed by the deter- 

 minants may seem to contradict the fact that many species are 

 undoubtedly constant in their characters, and have existed 

 without noteworthy change for a very long time. Among the 

 Cephalopods, the Nautilus has certainly existed since the Silurian 

 Age. The fact that in past ages the Nautili were extremely 

 numerous, whereas at present the few remaining species are 

 confined to the Indian and South Pacific Oceans, does not 

 alter the fact that their constancy of form has been preserved 

 throughout an irmnense period. How can we reconcile this 

 fact with the tendency to variation possessed by the deter- 

 minants ? 



Professor Emery of Bologna was, we think, the first to main- 

 tain that the tendency of certain variations to be carried to 

 excess was an important factor in the extinction of species. But 

 there seems no reason for supposing any variation to haVe an 

 irrepressible or irresistible power, incapable of being checked 

 by natural selection except by the extirpation of the whole 

 species. There are several remarkable cases of adaptation 

 known in which the particular variation stops just when the 

 adaptation is completed — that is to say, just when such varia- 

 tion attains the limits of biological value, and when any further 

 development would be harmful. These cases show that there 

 exists some relation of proportion between variation and 



