The difficulty as to higher animals is, however, 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 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 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 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 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 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 hypothesis, to escape admitting the independent origins of closely similar forms, It may be that they are both more frequent and more important than is generally thought.
That closely similar structures may arise without a genetic relationship has been lately well urged by Mr.Ray Lankester.[159] He has brought {153} this notion forward even as regards the bones of the skull in osseous fishes and in mammals.