130 THE GENESIS OF SPECIES. [Chap. 



liigliest organisms a tendency (mucli more niipetled and 

 rare in its manifestations) 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, under ordinary circum- 

 stances, a defmite limit to their variability, is largely su})- 

 j)()rted by facts brought forward by the zealous industry of 

 Mr. Darwin himself. It is unquestionable that the degrees 

 of variation which have been arrived at in domestic ani- 

 mals have been obtained more or less readily in a moderate 

 amount of time, but that further tlevelopment in certain 

 desired directions is in some a matter of extreme difliculty, 

 and in others appears to be all but, if not quite, an impos- 

 sibility. It is also unquestionable that the degree of di- 

 vergence ^vhich has been attained in one domestic species 

 is no criterion of the amount of divergence 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 by physical conditions, such, e. g., as those 

 which determine the greatest degree of speed possible to 

 any animal (of a given size) moving over the earth's sur- 

 face ; also it is said that the dilFerences in degree of change 

 shown by difl'erent domestic animals de})en(l in great meas- 

 ure upon the abundance or scarcity of individuals subjected 

 to man's selection, together with the varying diiection and 

 amount of his attention in dilTerent cases ; finally, it is said 

 that the changes found in Nature are within the limits to 

 which the variation of domestic animals extends — it being 

 the case, that when changes of a certain amount have oc- 

 curred to a species under nature, it becomes another species, 

 or sometimes two or more other species by ilivergent varia- 

 tions, each of these species being able again to vary and 

 diverge in any useful direction. 



But the fact of the rapidly-increasing difliculty found in 

 producing, by over such careful selection, any further ex- 



