CHAPTER IX.
LAWS OF TRANSMISSION BY INHERITANCE. ADAPTATION AND NUTRITION.
Distinction between Conservative and Progressive Transmission by Inheritance.—Laws of Conservative Transmission: Transmission of Inherited Characters.—Uninterrupted or Continuous Transmission.—Interrupted or Latent Transmission.—Alternation of Generations.—Relapse.—Degeneracy.—Sexual Transmission.—Secondary Sexual Characters.—Mixed or Amphigonous Transmission.—Hybrids.—Abridged or Simplified Transmission.—Laws of Progressive Inheritance: Transmission of Acquired Characters.—Adapted or Acquired Transmission.—Fixed or Established Transmission.—Homochronous Transmission (Identity in Epoch).—Homotopic Transmission (Identity in Part).—Adaptation and Mutability.—Connection between Adaptation and Nutrition.—Distinction between Indirect and Direct Adaptation.
In the last chapter we considered Transmission by Inheritance, one of the two universal vital activities of organisms, Adaptation and Inheritance, which by their interaction produce the different species of organisms, and we have endeavoured to trace this very mysterious vital activity to a more general physiological function of organisms, namely, to Propagation. This latter in its turn, like other vital phenomena of animals and plants, depends on physical and chemical relations. It is true they appear at times exceedingly complicated, but can nevertheless in reality be traced to simple mechanical causes—that is, to the relations of attraction and repulsion in the particles or molecules—in fact, to the motional phenomena of matter.
Now, before we turn our attention to the second function, the phenomenon of Adaptation or Mutability, which counteracts the Transmission by Inheritance, it seems appropriate first to cast one more glance at the various manifestations of Heredity, which we may perhaps even now denominate the “laws of transmission by inheritance.” Unfortunately, up to the present time very little has been done for this most important subject, either in zoology or in botany, and almost all we know of the different laws of inheritance is confined to the experiences of gardeners and farmers. It is not therefore to be wondered at, that on the whole these exceedingly interesting and important phenomena have not been investigated with desirable scientific accuracy, or reduced to the form of scientific laws. Accordingly, what I shall relate of the different laws of transmission are only some preliminary fragments taken out of the infinitely rich store which lies open to our inquiry.
We may first divide all the different phenomena of inheritance into two groups, which we may distinguish as the transmission of inherited characters, and the transmission of acquired characters; and we may call the former the conservative transmission, and the latter the progressive transmission by inheritance. This distinction depends upon the exceedingly important fact that the individuals of every species of animals and plants can transmit to their descendants, not only those qualities which they themselves have inherited from their ancestors, but also the peculiar, individual qualities which they have acquired during their own life. The latter are transmitted by progressive, the former by conservative inheritance. We have now first to examine the phenomena of conservative inheritance, that is, the transmission of such qualities as the organism has already received from its parents or ancestors. (Gen. Morph. ii. 180.)
Among the phenomena of conservative inheritance we are first struck by that which is its most general law, and which we may term the law of uninterrupted or continuous transmission. It is so universal among the higher animals and plants, that the uninitiated might overestimate its action and consider it as the only normal law of transmission by inheritance. This law simply consists in the fact that among most species of animals and plants, every generation is, on the whole, like the preceding—that the parents are as like the grandparents as they are like the children. “Like produces like,” as is commonly said, but more accurately “similar things produce similar things.” For, in reality, the descendants of every organism are never absolutely equal in all points, but only similar in a greater or less degree. This law is so generally known, that I need not give any examples of it.
The law of interrupted or latent transmission by inheritance, which might also be termed alternating transmission, is in a measure opposed to the preceding law. This important law appears principally active among many lower animals and plants, and manifests itself in contrast to the former in the fact that the offspring are not like their parents, but very dissimilar, and that only the third or a later generation becomes similar to the first. The grandchildren are like the grandparents, but quite unlike the parents. This is a remarkable phenomenon, and, as is well known, occurs also very frequently, though in a less degree, in human families. Every one of my readers doubtless knows some members of a family who, in this or that peculiarity, much more resemble the grandfather or grandmother than the father or mother. Sometimes it lies in bodily peculiarities, for example, features of face, colour of hair, size of body—sometimes in mental qualities, for example, temperament, energy, understanding—which are transmitted in this manner. This fact may be observed in domestic animals as well as in the case of man. Among the domestic animals most liable to vary—as the dog, horse, and ox—breeders very frequently find that the product by breeding resembles the grandparents far more than it does its own parental organism. If we express this general law and the succession of generations by the letters of the alphabet, then A = C = E, whilst B = D = F, and so on.