172 THE GENESIS OF SPECIES. [CHAP. 



if we could concede the probability of the independent 

 origin, in different situations, 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 organ- 

 isms, that the same forms can arise and manifest them- 

 selves independently. The difficulty as to higher animals 

 is, however, much greater, as (on the theory of evolution) 

 one acting force must always be the ancestral influences 

 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 done in the past. So that it is difficult to conceive that 

 individuals the ancestral 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 unequal s) 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 specific 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 insec- 

 tivorous 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 reptiles, and as the beak of the cuttle-fish or of 

 the tadpole resembles that of birds. We have already 

 seen (in Chapter III.) that it is impossible, upon any 



