166 THE GENESIS OF SPECIES. [CHAP. 



book that if other considerations render it improbable that 

 the manifestation of the successive forms of life has been 

 brought about by minute, indefinite, and fortuitous varia- 

 tions, then these facts as to geographical distribution in- 

 tensify that improbability, and are so far worthy of atten- 

 tion. 



All geographical difficulties of the kind would be evaded 

 if we could concede the probability of the independent 

 origin, in different localities, of the same organic forms in 

 animals high in the scale of nature. Similar causes must 

 produce similar results, and new reasons have been lately 

 adduced for believing, as regards the lowest organisms, 

 that the same forms can arise and manifest themselves inde- 

 pendently. The difficulty as to higher animals is, how- 

 ever, much greater, as (on the theory of evolution) one 

 acting force must always be the ancestral history in each 

 case, and this force must always tend to go on acting in the 

 same groove and direction in the future as it has in the past. 

 So that it is difficult to conceive that individuals, the ances- 

 tral history of which is very different, can be acted upon by 

 all influences, external and internal, in such diverse ways 

 and proportions that the results (unequals being added to 

 unequals) shall be equal and similar. Still, though highly 

 improbable, this cannot be said to be impossible ; and if 

 there is an innate law of any kind helping to determine spe- 

 cific evolution, this may more or less, or entirely, neutralize 

 or even reverse the effect of ancestral habit. Thus, it is quite 

 conceivable that a pleurodont lizard might have arisen in 

 Madagascar in perfect independence of the similarly-formed 

 American lacertilia : just as certain teeth of carnivorous 

 and insectivorous marsupial animals have been seen most 

 closely to resemble those of carnivorous and insectivorous 

 placental beasts ; just as, again, the paddles of the Cetacea 

 resemble in the fact of a multiplication in the number of 

 the phalanges, the many-jointed feet of extinct marine rep- 



