What, then, is an organic species? It may be defined as a group of organisms endowed with the hardihood necessary to survive and propagate themselves under natural conditions (i.e. in the wild state), exhibiting a common inheritable type, differing from one another by no major germinal difference, perfectly interfertile with one another, but sexually incompatible with members of an alien specific group, in such wise that they produce hybrids wholly, or partially, sterile, when crossed with organisms outside their own specific group.
David Starr Jordan has wisely called attention to the requisite of viability and survival under natural conditions. “A species,” he says, “is not merely a form or group of individuals distinguished from other groups by definable features. A complete definition involves longevity. A species is a kind of animal or plant which has run the gauntlet of the ages and persisted.... A form is not a species until it has ‘stood.’” (Science, Oct. 20, 1922, p. 448.)
Sexual (gametic) incompatibility as a criterion of specific distinction, presupposes the bisexual or biparental mode of reproduction, namely, syngamy, and is therefore chiefly applicable to the metista, although, if the view tentatively proposed by the protozoölogist, E. A. Minchin, be correct, it would also be applicable to the protista. According to this view, no protist type is a true species, unless it is maintained by syngamy (i.e. bisexual reproduction)—“Not until syngamy was acquired,” says Minchin, “could true species exist among the Protista.” (“An Introduction to the Study of the Protozoa,” p. 141.)
To return, however, to the metista, the horse (Equus caballus) and the ass (Equus asinus) represent two distinct species under a common genus. This is indicated by the fact that the mule, which is the hybrid offspring of their cross, is entirely sterile, producing no offspring whatever, when mated with ass, horse, or mule. Such total sterility, however, is not essential to the proof of specific differentiation; it suffices that the hybrid be less fertile than its parents. As early as 1686, sterility (total or partial) of the hybrid was laid down by John Ray as the fundamental criterion of specific distinction. Hence Bateson complains that Darwinian philosophy flagrantly “ignored the chief attribute of species first pointed out by John Ray that the product of their crosses is frequently sterile in a greater or lesser degree.” (Science, Jan. 20, 1922, p. 58.)
Accordingly, the sameness of type required in members of the same species refers rather to the genotype, that is, the sum-total of internal hereditary factors latent in the germ, than to the phenotype, that is, the expressed somatic characters, viz. the color, structure, size, weight, and all other perceptible properties, in terms of which a given plant or animal is described. Thus it sometimes happens that two distinct species, like the pear-tree and the apple-tree, resemble each other more closely, as regards their external or somatic characters, than two varieties belonging to one and the same species. Nevertheless, the pear-tree and the apple-tree are so unlike in their germinal (genetic) composition that they cannot even be crossed.
According to all theories of transformism, new species arise through the transformation of old species, and hence evolutionists are at one in affirming the occurrence of specific change. When it comes, however, to assigning the agencies or factors, which are supposed to have brought about this transmutation of organic species, there is a wide divergence of opinion. The older systems of transformism, namely, Lamarckism and Darwinism, ascribed the modification of organic species to the operation of the external factors of the environment, while the later school of orthogenesis attributed it to the exclusive operation of factors residing within the organism itself.
Lamarckism, for example, made the formation of organs a response to external conditions imposed by the environment. The elephant, according to this view, being maladjusted to its environment by reason of its clumsy bulk, developed a trunk by using its nose to compensate for its lack of pliancy and agility. Here the use or function precedes the organ and molds the latter to its need. Darwinism agrees with Lamarckism in making the environment the chief arbiter of modification. Its explanation of the elephant’s trunk, however, is negative rather than positive. This animal, it tells us, developed a trunk, because failure to vary in that useful direction would have been penalized by extermination.
Wilson presents, in a very graphic manner, the appalling problem which confronts evolutionists who seek to explain the adaptations of organisms by means of environmental factors. Referring, apparently, to Henderson’s “Fitness of the Environment,” he says: “It has been urged in a recent valuable work ... that fitness is a reciprocal relation, involving the environment no less than the organism. This is both a true and suggestive thought; but does it not leave the naturalist floundering amid the same old quicksands? The historical problem with which he has to deal must be grappled at closer quarters. He is everywhere confronted with specific devices in the organism that must have arisen long after the conditions of environment to which they are adjusted. Animals that live in water are provided with gills. Were this all, we could probably muddle along with the notion that gills are no more than lucky accidents. But we encounter a sticking point in the fact that gills are so often accompanied by a variety of ingenious devices, such as reservoirs, tubes, valves, pumps, strainers, scrubbing brushes, and the like, that are obviously tributary to the main function of breathing. Given water, asks the naturalist, how has all this come into existence and been perfected? The question is an inevitable product of our common sense.” (Smithson. Inst. Rpt. for 1915, p. 405.)
Impressed with the difficulty of accounting for the phenomena of organic adaptation by means of the far too general and unspecific influence of the environment, the orthogenetic school of transformism inaugurated by Nägeli, Eimer, and Kölliker repudiated this explanation, and sought to explain organic evolution on the sole basis of internal factors, such as “directive principles,” or germinal determinants. According to this conception, the elephant first developed his trunk under the drive of some internal agency, and afterwards sought out an environment in which the newly-developed trunk would be useful. In other words, orthogenesis makes the organ precede the function, and is therefore the exact reverse of Lamarckism.
Evolutionists in general, as we have said, regard our present plants and animals as the modified progeny of earlier forms, understanding by “modified” that which is the product of a trans-specific, as distinguished from a varietal or intra-specific, change. To substantiate the claim that changes of specific magnitude have actually taken place, they appeal to two principal kinds of evidence, namely: (a) empirical evidence based on such variations as are now observed to occur among living organisms; (b) inferential evidence, which aposterioristically deduces the common ancestry of allied organic types from their resemblances and their sequence in geological time. Hence, if we omit as negligible certain subsidiary arguments, the whole evidence for organic evolution may be summed up under three heads: (1) the genetic evidence grounded on the facts of variation; (2) the zoölogical evidence based on homology, that is, on structural resemblance together with all further resemblances (physiological and embryological), which such similarity entails; (3) the palæontological evidence which rests on the gradual approximation of fossil types to modern types, when the former are ranged in a series corresponding to the alleged chronological order of their occurrence in the geological strata. It is the bearing of recent genetical research upon the first of these three lines of evidence that we propose to examine in the present chapter, an objective to which a brief and rather eclectic historical survey of evolutionary thought appears to offer the easiest avenue of approach.