Зображення сторінки

A careful observer of animal life who has long resided in South Africa, explored the interior, and lived in the giraffe country, has assured the Author that the giraffe has powers of locomotion and endurance fully equal to those possessed by any of the other Ungulata of that continent. It would seem, therefore, that some of these other Ungulates ought to have developed in a similar manner as to the neck, under pain of being starved, when the long neck of the giraffe was in its incipient stage.

To this criticism it has been objected that different kinds of animals are preserved, in the struggle for life, in very different ways, and even that “high-reaching” may be attained in more modes than one—as, for example, by the trunk of the elephant. This is indeed true, but then none of the African Ungulata 1 have, nor do they appear ever to have had, any proboscis whatsoever ; nor have they acquired such a development as to allow them to rise on their hind limbs and graze on trees in kangaroo-attitude, nor a power of climbing, nor, as far as known, any other modification tending to compensate for the comparative shortness of the neck. Again, it may perhaps be said that leaf-eating fornis are exceptional, and that therefore the struggle to attain high branches would not affect many Ungulates. But surely, when these severe droughts necessary for the theory occur, the ground vegetation is supposed to be exhausted; and, indeed, the giraffe is quite capable of feeding from off the ground. So that, in these cases, the other Ungulata must have taken to leaf-eating or have starved, and thus must have had any accidental long-necked varieties favoured and preserved exactly as the long-necked vari

1 The elephants of Africa and India, with their extinct allies, constituto the order Protoscidea, and do not belong to the Ungulata.

He says,

ties of the giraffe are supposed to have been favoured and preserved.

The argument as to the different modes of preservation has been very well put by Mr. Wallace, in reply to the objection that "colour, being dangerous, should not exist in nature.” This objection appears similar to the one here urged; as it is here said that a giraffe neck being needful, there should be many animals with it, while the objector noticed by Mr. Wallace says, “A dull colour being needful, all animals should be so coloured.And Mr. Wallace shows in reply how porcupines, tortoises and mussels, very hard-coated bombardier beetles, stinging insects and nauseous-tasted caterpillars, can afford to be brilliant by the various means of active defence or passive protection they possess, other than obscure coloration. « The attitudes of some insects may also protect them, as the habit of turning up the tail by the harmless rovebeetles (Staphylinidæ, no doubt leads other animals, besides children, to the belief that they can sting. The curious attitude assumed by sphinx caterpillars is probably a safeguard, as well as the blood-red tentacles which can suddenly be thrown out from the neck by the caterpillars of all the true swallow-tailed butterflies.”

But, because many different kinds of animals can elude the observation or defy the attack of enemies in a great variety of ways, it by no means follows that there are any similar number and variety of ways for attaining vegetable food in a country where all such food, other than the lofty branches of trees, has been for a time destroyed. In such a country we have a number of vegetable-feeding Ungulates, all of which present minute variations as to the

1 See “Natural Selection,” pp. 60—75.

length of the neck. If, as Mr. Darwin contends, the natural selection of these favourable variations has alone lengthened the neck of the giraffe by preserving longnecked individuals during droughts; similar variations, . in other similarly-feeding forms, ought similarly to have been preserved, and so have lengthened the neck of such other Ungulates by similarly preserving them during the same droughts.

(2.) It may be also objected, that the power of reaching upwards, acquired by the lengthening of the neck and legs, must have necessitated a considerable increase in the entire size and mass of the body (larger bones requiring stronger and more voluminous muscles and tendons, and these again necessitating larger nerves, more capacious blood-vessels, &c.), and it is very problematical whether the disadvantages thence arising would not, in times of scarcity, more than counterbalance the advantages. For a considerable increase in the supply of food would be requisite on account of this increase in size and mass, while at the same time there would be a certain decrease in strength; since, as Mr. Herbert Spencer says:1 “It is demonstrable that the excess of absorbed over expended nutriment must, other things equal, become less as the size of an animal becomes greater. In similarly-shaped bodies, the masses vary as the cubes of the dimensions; whereas the strengths vary as the squares of the dimensions." Supposing a creature which a year ago was one foot high, has now become two feet high, while it is unchanged in proportions and structure-what are the necessary concomitant changes that have taken place in it? It is eight times as heavy; that is to say, it has to resist eight times the strain which gravitation puts on its struc

1 Principles of Biology,” vol. i. p. 122.

[ocr errors]

ture; and in producing, as well as in arresting, every one of its movements, it has to overcome eight times the inertia. Meanwhile, the muscles and bones have severally increased their contractile and resisting powers, in proport tion to the areas of their transverse sections; and hence are severally but four times as strong as they were. Thus, while the creature has doubled in height, and while its ability to overcome forces has quadrupled, the forces it has to overcome have grown eight times as great. Hence, to raise its body through a given space, its muscles have to be contracted with twice the intensity, at a double cost of matter expended." Again, as to the cost at which nutriment is distributed through the body, and effete matters removed from it, “ Each increment of growth being added at the periphery of an organism, the force expended in the transfer of matter must increase in a rapid progressiona progression more rapid than that of the mass.

There is yet another point. Vast as may have been the time during which the process of evolution has continued, it is nevertheless not infinite. Yet, as every kind, on the Darwinian hypothesis, varies slightly but indefinitely in every organ and every part of every organ, how very generally must favourable variations as to the length of the neck have been accompanied by some unfavourable variation in some other part, neutralizing the action of the favourable one, the latter, moreover, only taking effect during these periods of drought! How often must individuals, favoured by a slightly increased length of neck, have failed to enjoy the elevated foliage which they had not strength or endurance to attain ; while other individuals, exceptionally robust, could struggle on yet further till they arrived at vegetation within their reach.

However, allowing this example to pass, many other instances will be found to present great difficulties.

Let us take the cases of mimicry amongst Lepidoptera and other insects. Of this subject Mr. Wallace has given a most interesting and complete account, showing in how many and strange instances this superficial resemblance by one creature to some other quite distinct creature acts as a safeguard to the first. One or two instances must here suffice. In South America there is a family of butterflies, termed Heliconido, which is very conspicuously coloured and slow in flight, and yet the individuals abound in prodigious numbers, and take no precautions to conceal themselves, even when at rest during the night. Mr. Bates (the author of the very interesting work The Naturalist on the River Amazons," and the discoverer of “Mimicry") found that these conspicuous butterflies had a very strong and disagreeable odour; so much so that any one handling them and squeezing thein, as a collector must do, has his fingers stained and so infected by the smell as to require time and much trouble to remove it.

It is suggested that this unpleasant quality is the cause of the abundance of the Heliconidæ; Mr. Bates and other observers reporting that they have never seen them attacked by the birds, reptiles, or insects which prey upon other Lepidoptera.

Now it is a curious fact that very different South American butterflies put on, as it were, the exact dress of these offensive beauties and mimic them even in their mode of flight In explaining the mode of action of this protecting re

1 See “Natural Selection," chap. iii. p. 45.


« НазадПродовжити »