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infantile characteristics into the period of man. hood.
Now suppose any number of individual animals or plants to set out in life, from the egg or the seed, as nearly as possible alike, not merely of the same species but of the same variety, and almost indistinguishable from each other. Let a part of the whole number go on through certain stages of growth to maturity, and then in turn reproduce their kind. Suppose the remain. ing portion of the original number to continue on in their development to a point beyond that reached by the former portion, so that their adult condition differs from that of the less-developed individuals in some such way as the salamander differs from the axolotl or the Caucasian from the negro. Here we have an example of the formation of a new genus, species, or variety, as the case may be, by what Hyatt and Cope call acceleration. In speaking of the Ammon. ites, a most important group of fossil shell-fish, Professor Hyatt thus defines the law of acceleration :
“The young of higher species are thus constantly accelerating their development and reducing to a more and more embryonic condition the stages of growth corresponding to the adult periods of preceding or lower species." Retardation, a process exactly the reverse of acceleration, would produce new species or other groups by a kind of degeneration or arrested development, marked by the omission of the later steps in the evolution of the individual, and the assumption of the reproductive function at a point somewhat short of the development reached by the parent-species. Evidence in abundance of the generality of the occurrenco of both acceleration and retardation as factors in species-making, has already been collected.
For evidence to prove the fact of inheritance of acquired characteristics it must here suffice to refer the reader once more to the instances given on pp. 32 to 40, inclusive, adding only the fact, discovered by Brown-Sequard, that epilepsy artificially induced in guinea-pigs is inherited by their offspring.1
Turning, now, to the subject of crossing, a few facts may be given in regard to its occurrence, and its influence on the production of species. It is hardly possible to account for the extraordinary differences in form, size, disposition, and habits of the domestic dog, without supposing that the present races are descended, not from one, but from several, wild species, which have crossed and recrossed till the many known races have resulted. In fact, no wild species can be pointed out as at all closely resembling, even a majority of the domestic kinds. Add to this the fact that the latter may be crossed with wild dogs of different species, with wolves, with jackals, with foxes even, and in all cases but the last (if not in that), the offspring of the cross breed freely with either parent species, and sometimes with each other. It is true, domestication often seems to be essential to the removal of a sterility frequently noticed when wild animals are crossed; but there are abundant instances of the fertility of the product of such crosses, either with one of the parent species, or with each other. The domestic cat also crosses freely with wild species, probably with not less than nine different ones ; 3 and the result of the crossing is to produce mongrels which generally breed freely.
1 The reader who cares to make himself familiar with the details of the arguments of the Neo-Lamarckian school of evolutionists - a school which attributes the origin of species and other groups mainly to the variation of the individual, brought about by the operation of external and internal causes and perpetuated by heredity — will find their case most ably set forth in Prof. Edward D. Cope's Origin of the Fittest, or in a less detailed and more popular form in Prof. H. W. Conn's Evolution of To-day.
1 Darwin, Animals and Plants, etc., i. pp. 21-34. 2 Haeckel, Natural History of Creation, vol. i. pp. 147, 148. 8 Darwin, Animals and Plants, etc., i. pp. 46, 47.
How readily plants hybridize, one race with another, is well known to all who have had much to do either with farming or with gardening. Pure seed cannot be raised from sweetcorn planted near field-corn, nor from melous grown near squashes or pumpkins. Broomcorn and sorghum, too, are notoriously likely to hybridize when grown in each other's vicinity. The hybridizing is in all cases brought about by the fertilizing-dust (pollen) froin one plant falling on the tip of the seed-producing organ (pistil) of the other plant, and there sending out little tubes which penetrate the whole length of the pistil, and cause the immature seeds (ovules) contained within its base to grow and ripen. The transference of the pollen from one plant to another is accomplished by the wind, or by insects travelling from flower to flower. The same process is often carried on artificially, with the intention of producing new varieties, as in the production of the well-known Rogers Hybrid grapes. In the cases just cited, the hybridizing takes place between varieties of the same species; but there are plenty of instances of its occurrence between different species, resulting in the production of perfectly fertile seed. So
1 By race an extremely well-marked variety is meant. 2 Darwin, Animals and Plants, etc., ii. pp. 107-112.
common is hybridism in certain wild genera of plants, notably among the oaks and the asters, that it is impossible to decide, in regard to certain forms, whether we should call them species, varieties, or hybrids. In view, then, of the known facts, – of which the ones just cited form but an insignificant fraction, - it seems at least to be altogether probable that many species of animals, and still more species of plants, have originated by crossing and by hybridization.
So far, in this chapter, only the operation of more or less perfectly understood causes of variation has been considered. It is easy enough to see how even so remarkable a change as that of the axolotl into the salamander may be brought about by its removal from the water and the consequent loss of its gills. But the instances in which changes are produced by causes which are imperfectly or not at all understood, are far more numerous and striking than the better-understood kind. The very real though almost indescribable alteration which Europeans undergo, after a generation or two, on being transplanted to this continent, forms an excellent example of the all-pervading effect which may result from causes too minute to be entirely separated from one another, and too