IGG THE GENESIS OF SPECIES. [Chap. 



book that if oilier considerations render it iin])rol)able tliut 

 the manifestation ^f the successive forms of life has been 

 brought about by minute, indefinite, and fortuitous varia- 

 tions, tlien these facts as to geograpliical distribution in- 

 tensify that im})robability, and are so far worthy of atten- 

 tion. 



All geographical difhculties of tlie kind would be evaded 

 if we could concede the probability of tlie independent 

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

 animals high in the scale of nature. Similar causes nmst 

 produce similar results, and new reasons have been lately 

 adduced for believing, as regards the lowest oryanlsnis, 

 that the same forms can arise and manifest themselves inde- 

 pendently. The dilliculty 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 ditlicult to conceive that individuals, the ances- 

 tral history of which is very different, can ho 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 heli)ing to determine spe- 

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

 or even reverse the efl'ect of ancestral habit. Thus, it is quite 

 conceivable that a plenrodont 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- 



