334 



HABIT AND INTELLIGENCE. 



[chap. 



Effect of 

 isolation 

 iu pro- 

 ducing 

 change. 



The 

 largest 

 areas pro- 

 duce the 

 most 

 dominant 

 species. 



General 

 effect of 

 geological 

 conditions. 



time enoiigli is allowed variation is perfectly indefinite in 

 amount, at least within the limits of the same fundamental 

 form. Isolation alone appears to produce change. If a 

 small colony of any species whatever were to be isolated 

 from its kind, it can scarcely be doubted that after some 

 generations it would come to constitute a distinct variety. 

 Such cases have occurred under domestication;^ and they 

 must have been of constant occurrence in nature whenever 

 some of the individuals of a species have been isolated 

 from the rest by the formation of mountain or sea barriers. 

 But it is on the largest areas obviously that there will be 

 the largest number of chances of favourable variations 

 occurring ; and there consequently those species will be 

 perfected which will be most successful iu the " struggle 

 for existence." 2 Thus European and Asiatic species of 

 plants are now overspreading Australia and New Zealand, 

 while the Australian and New Zealand species, which have 

 been introduced as garden plants, have little tendency to 

 become wild in Europe. No doubt the most favourable 

 state of things for the promotion of variation and advance- 

 ment is that which geological conditions have produced, — 

 constant, but generally slow changes in the conditions of 

 life ; general severe competition, with many isolated areas 

 in which it is less intense ; colonies frequently shut off 

 from the parent stock, and left alone to vary or to remain 

 unchanged ; and species which had been matured in sepa- 

 rate regions suddenly brought into competition by the 

 removal of barriers. Every geological change will tend to 



1 " Youatt gives an excellent illustration of the effects of a course of 

 selection, which may be considered as unconsciously followed, in so far that 

 the breeders could never have expected, or even have wished, to produce 

 the result which ensued — namely, the production of two distinct strains. 

 The two flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess, as 

 Mr. Youatt remarks, ' have been pui'ely bred from the original stock of 

 Mr. Bakewell for upwards of fifty years. There is not a suspicion existing 

 in the mind of any one at all acquainted with the subject, that the o-svner 

 of either of them has deviated in any one instance from the pure blood of 

 Mr. Bakewell's flock, and yet the difference between the sheep possessed 

 by these two gentlemen is so great, that they have the appearance of 

 being quite different varieties.' " (Darwin's Origin of Species, p. 37.) 



2 Ibid. p. 119. 



