The experiments of M. Brown Séquard on guinea pigs, quoted by Mr. Darwin,[298] suggest that the form of the serpent may be due to its having lost its legs by successive accidents in squeezing through narrow places, and that the wounds having been followed by disease, the creature may have bitten the limbs off, in which case the loss might have been very readily transmitted to offspring; the animal would accordingly take to a sinuous mode of progression that would doubtless in time elongate the body still further. M. Brown Séquard "carefully recorded" thirteen cases, and saw even a greater number, in which the loss of toes by guinea pigs which had gnawed their own toes off, was immediately transmitted to offspring. Accidents followed by disease seem to have been somewhat overlooked as a possible means of modification. The missing forefinger to the hand of the potto[299] would appear at first sight to have been lost by some such mishap. Returning to Lamarck, we find him saying:—
"Even in the lifetime of a single individual we can see organic changes in consequence of changed habits. Thus M. Tenon has constantly found the intestinal canal of drunkards to be greatly shorter than that of people who do not drink. This is due to the fact that habitual drunkards eat but little solid food, so that the stomach and intestines are more rarely distended. The same applies to people who lead studious and sedentary lives. The stomachs of such persons and of drunkards have little power, and a small quantity will fill them, while those of men who take plenty of exercise remain in full vigour and are even increased."[300]
It becomes now necessary to establish the converse proposition, namely that:—
"The frequent use of an organ increases its power; it even develops the organ itself, and makes it acquire dimensions and powers which it is not found to have in animals which make no use of such an organ.
"In support of this we see that the bird whose needs lead it to the water, in which to find its prey, extends the toes of its feet when it wants to strike the water, and move itself upon the surface. The skin at the base of the toes of such a bird contracts the habit of extending itself from continual practice. To this cause, in the course of time, must be attributed the wide membrane which unites the toes of ducks, geese, &c. The same efforts to swim, that is to say, to push the water for the purpose of moving itself forward, has extended the membrane between the toes of frogs, turtles, the otter, and the beaver."[301]
[This is taken, I believe, from Dr. Darwin or Buffon, but I have lost the passage, if, indeed, I ever found it. It had been met by Paley some years earlier (1802) in the following:—
"There is nothing in the action of swimming as carried on by a bird upon the surface of the water that should generate a membrane between the toes. As to that membrane it is an action of constant resistance.... The web feet of amphibious quadrupeds, seals, otters, &c., fall under the same observation."[302]]
"On the other hand those birds whose habits lead them to perch on trees, and which have sprung from parents that have long contracted this habit, have their toes shaped in a perfectly different manner. Their claws become lengthened, sharpened, and curved, so as to enable the creature to lay hold of the boughs on which it so often rests. The shore bird again, which does not like to swim, is nevertheless continually obliged to enter the water when searching after its prey. Not liking to plunge its body in the water, it makes every endeavour to extend and lengthen its lower limbs. In the course of long time these birds have come to be elevated, as it were, on stilts, and have got long legs bare of feathers as far as their thighs, and often still higher. The same bird is continually trying to extend its neck in order to fish without wetting its body, and in the course of time its neck has become modified accordingly.[303]
"Swans, indeed, and geese have short legs and very long necks, but this is because they plunge their heads as low in the water as they can in their search for aquatic larvæ and other animalcules, but make no effort to lengthen their legs."[304]
This too is taken from some passage which I have either never seen or have lost sight of. Paley never gives a reference to an opponent, though he frequently does so when quoting an author on his own side, but I can hardly doubt that he had in his mind the passage from which Lamarck in 1809 derived the foregoing, when in 1802 he wrote § 5 of chapter xv. and the latter half of chapter xxiii. of his 'Natural Theology.'