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stones are in part composed had been dropped from the hats of pilgrims returning from the Holy Land. Such quibbling, or the assumption that different forms of snail-shell might have been the result of separate acts of creation if they had been found in different places, but are to be regarded as of common descent if found in the deposit of a single lake, may do for professed wits, but have no flavor of science about them.
But how much more rational is it to believe that the animals and plants of all the geological ages have from time to time been killed off, and new ones made out of earth and air, and water, to take their place, than to believe this of the Steinheim snails?
With this brief account of the subject of variation, the present chapter must close, not because there are not examples enough in illustration of the matter to fill many volumes, but because all that it is necessary to do in this place is to show that species may vary till they become changed into other species.? Of this fact the case of the Steinheim deposits or that of the Arietidæ pages 150–155, is in itself in proof; but such instances as are afforded by the oak genus, with its two hundred doubtful species out of three hundred in all, form even more convincing evidence of the irresistible tendency of animals and plants to vary. How sudden and profound such variations may sometimes be is shown by cases like that of certain brine-shrimps, to which I shall refer more fully in the coming chapter. These animals, under changed conditions, were found to vary, not merely from one species into another, but from one genus
1 For a very recent and full account of the variation of wild species at the present day, see Chapter III. of Alfred Russell Wallace's “ Darwinism," Macmillan & Co., London and New York, 1889.
into another. The question as to whether the time or the manner of beginning of any new species has ever been discovered, asked at the beginning of this chapter, cannot be better answered than in the language of Professor Huxley, than whom there is no greater living English zoologist:
“On the evidence of paleontology [the science which treats of the life of previous geological ages], the evolution of many existing forms of animal life from their predecessors is no longer an hypothesis, but an historical fact: it is only the nature of the physiological factors to which that evolution is due that is still open to discussion." 1
1 Article on Evolution in the Encyclopædia Britannica. It may be as well to explain here that different authors use the terms “development theory," "evolution" or "organic evolution,” and “theory of descent,” to mean about the same thing. Darwinism is properly a term of more restricted meaning, as will be seen from Chap. IV.
I shall in the next chapter endeavor to ex. plain some of the most important opinions that are held concerning the nature of the “physiological factors ” of which Professor Huxley speaks.
CAUSES GIVING RISE TO VARIATIONS.
No one as yet entirely understands, and per
haps no one ever will understand, what gives rise to the likenesses on the one hand, and the differences on the other hand, that may always be noticed when the parent animal or plant is compared with its offspring. That like produces like is a common enough saying. But the resemblance is never, or almost never, perfect; that is, the animal or plant differs from its parent in size, shape, relative proportion of parts, or in color and markings; while, besides these structural differences, there are differences in function, manifesting themselves in varied habits, improved or impaired health and strength, or, at any rate, in lengthened or shortened life. But though we must, for the present, dismiss, with no explanation, the great general fact of variation in all offspring of all organisms, yet naturalists have been able to lay down some laws which govern variation; and these it is the
object of the present chapter briefly to state. In this connection, too, it will be necessary to say something of the changes which may be produced in the individual animal or plant during its own lifetime.
In substance it may be asserted, that any change in the circumstances, i.e., the sum total of the influences affecting any organism, will be likely to work some alteration in that organism or its descendants, or in both.
Chief among the external influences affecting the existence of any being are the food, soil, climate, supply of light, and the attacks of enemies, either of its own or of other species. To these, in the case of the higher animals, must be added a list of internal influences, arising mainly from the habits of life and the disposition of the animal itself.
“ Instances could be given of similar varieties being produced from the same species under external conditions of life as different as can well be conceived, and, on the other hand, of dissimilar varieties being produced under apparently the same external conditions.” 1
Lengthening or shortening the period of life before birth is to be reckoned as another factor, often, among the higher animals, of much im
1 Darwin's Origin of Species, p. 107.