130 THE GENESIS OF SPECIES. [Chap. 



damp and dark jtlace." This indeed is scarcely credible, 

 but it does not quite follow that the forms developed are 

 necessarily the same species, if, as Dr. Bastian seems to 

 show, thoroui^ddy different and distinct ol'ganic forms ^ can 

 be evolved one from another by modifying tlie conditions. 

 The last-named oljserver has brought forward arguments 

 and facts from wliii-li it would appear that such definite, 

 sudden, and considerable transformations may take place 

 in the lowest organisms. If such is really the case, we 

 might expect, a priori, to find in the highest organisms a 

 tendency (much more impeded and rare in its manifesta- 

 tions) to similarly appreciable and sudden changes, under 

 certain stimuli ; but a tendency to continued stability, 

 under normal and ordinary conditions. The proposition 

 that species have, normally, a definite limit to their vari- 

 ability, is largely supported by facts brought forward 

 by the zealous industry of ]\Ir. Darwin himself. It is 

 un([uestionable that the degrees of variation which have 

 been arrived at in domestic animals have been obtained 

 more or less readily in a moderate amount of time; but 

 that further development in certain desired directions 

 is in some a matter of extreme ditticultv, and in others 

 appears to be all but, if not quite, an impossibility. 

 It is also unquestionable that the degree of divergence 

 which has been attained in one domestic species is no 

 criterion of the amount of diveigence which has been 

 attained in another. It is contended on the other side, that 

 we have no evidence of any limits to variation other than 

 those imposed liv i»hysical conditions, such, c.r/., as those 

 which determine the greatest degree of speed possible to 



^ See Nature, June and July 1870, Xos. 35, 36, and 37, pp. 170, 193, 

 and 219. 



