Now, it is clear that Darwin's being necessitated particularly to note that the rule that advantage results from crossing obtains even in the cases of cattle and sheep, implies that comparatively little good accrues to the offspring from the crossing of the breeds of either of those animals. This shows, as the varieties of the sheep and cow are convergent in character, that the less divergent the varieties the less is the good attendant on crossing. The converse, that the more divergent the varieties the greater the good, is plainly seen in the case of the pigeon, of which the varieties are manifestly and confessedly the most divergent. The following assertions are unequivocal proof of our view:
"All the domestic races pair readily together, and, what is equally important, their mongrel offspring are perfectly fertile. To ascertain this fact, I made many experiments, which are given in the note below; and recently Mr. Tegetmeier has made similar experiments with the same result. The accurate Neumeister asserts that when dovecots are crossed with pigeons of any other breed the mongrels are extremely fertile and hardy. MM. Boitard and Corbie affirm, after their great experience, that with crossed pigeons, the more distinct the breeds, the more productive are their mongrel offspring." (Page 236, vol i., American edition.)
Mere mention of crossing in connection with our theory would, we conceive, suffice. But if any doubts have been entertained of the conclusiveness of the proofs furnished by the law, or of the competency of the theory of reversion to account for the good resulting from crossing, they are now surely dissipated by the evidence adduced from Darwin. The law of crossing which we propound is no ultimate law. It fulfils every requirement of a derivative law. The good which flows from crossing varies in degree in different animals, as is well known. This is quite explicable upon our theory; and the amount of good accruing to the offspring from the union of two given varieties, is even susceptible of prevision. Crossing per se does not produce the increased good; it is attributable to the lack of full and proportionate development. Of course, for increased good to result, each of the crossed animals must contribute to the formation of the offspring a part or parts which the other lacks. We have, then, given what Darwin's law, being purely empirical, is utterly incompetent to do—a rational and consistent interpretation of the variations in the quantity of the effects. Logic requires no greater proofs of a theory than those which we have here adduced.
Darwin has informed us, in his late invaluable work, that crossing induces the appearance of new characters. Great stress is laid upon this fact by several writers, and some of them, among whom Pallas is conspicuous, have even gone so far as to ascribe variability exclusively to crossing. The theory of reversion furnishes a rational explanation of the appearance of these characters. We do not allude merely to the fact that their reversion is more probable than their evolution; for Darwin inclines to this opinion rather than to the contrary one. On page 264, vol. ii., after demurring to the conception that variability is solely induced by crossing, he says:
"Nevertheless, it is probable that the crossing of two forms, when one or both have long been domesticated or cultivated, adds to the variability of the offspring, independently of the commingling of the characters derived from the two parent forms; and this implies that new characters actually arise. But we must not forget the facts advanced in the thirteenth chapter, which clearly prove that the act of crossing often leads to the reappearance or reversion of long-lost characters; and in most cases, it would be impossible to distinguish between the reappearance of ancient characters and the first appearance of new characters. Practically, whether new or old, they would be new to the breed in which they reappeared."
But there is another factor subserving evolution, to which we particularly allude. This is correlation, which we have seen reason to conclude exists, not only between different growths, but also between different centres of growth. Now, when a cross ensues, the offspring generally acquires from each parent a character or characters which the other lacks. The union of these characters strengthens the centres to which they are joined, and also all the centres of which the related parts are developed. By correlation, the centre to which these centres are most closely allied becomes more firmly established. The more firm establishment of this centre, then, induces the development of its formerly connected parts. These parts are the characters consequent upon crossing.
If, as we maintain, the proofs furnished by crossing are conclusive, then the phenomena of close interbreeding must be proofs amounting to demonstration. For the law of close interbreeding, which is the converse of that of crossing, also holds good; is, if possible, more in accordance with the theory of reversion; is also susceptible of resolution into the law of proportionate development; and, being a derivative law upon our theory, fully accounts for all the variations in the quantity of the effects. The different data, moreover, esteemed so mutually inconsistent, of those who concur in and of those who demur to Darwin's law of close interbreeding, can be shown, by the light furnished by the hypothesis of proportionate development, to be perfectly congruous. If we can prove, then, that our law of close interbreeding, founded upon the facts furnished by Darwin, is capable of all this, we shall have fulfilled our promise to place our theory beyond the reach of cavil.
As has been more than once asserted, our views necessitate the conclusion that a multiplicity of divergent varieties implies the loss in each of what constitute the peculiar characteristics of the others. The circumstance that some few varieties are distinguished by the possession of negative features, but slightly modifies this conclusion. Now, it is clear to the comprehension of every one who is likely to have followed us this far, that, as the loss of any part or character is deleterious, the pairing of the members of a variety would tend to aggravate the evil consequent on the absence of the peculiar characters of the other varieties.
Quite in harmony with this view is the following assertion, one of a vast number of a similar kind made by Darwin: "The consequences of close interbreeding, carried on for too long a time, are, as is generally believed, loss of size, constitutional vigor, and fertility, sometimes accompanied by a tendency to malformation." (Page 115, vol. ii.)
Now, according to our theory, the evil effects of close interbreeding must be proportionate to the divergence of character; or, rather, to the disproportionate development which divergence involves. Darwin admits that different species of animals are differently affected by the same degree of interbreeding. Among species of which the varieties are divergent, the pigeon and fowl are preëminently conspicuous. Here, then, we must look for the greatest evil effects from the interbreeding of the members of the varieties. The facts fail not to realize our anticipations. No writers have expressed so strong a conviction of the impossibility of long-continued interbreeding as Sir J. Sebright and Andrew Knight, who have paid the most attention to the breeding of the fowl and pigeon. Darwin gives us, as the result of his wide experience and extensive research, the following opinion: