2. Through a condition of panmixia the general average maintained by selection is reduced to the birth mean in one generation (ontogenetic—Romanes, Lankester, Lloyd Morgan, Weismann) to the greatest possible degeneration in succeeding generations (phylogenetic—Weismann), or but little below the birth average of the first generation (Weismann’s later view, Romanes, Morgan, Lankester).
3. Through natural selection (reversed), the struggle of persons, the organ may be caused to degenerate either (A) by the migration of persons with highly developed eyes from the colony living in the dark (Lankester), or (B) through economy of weight and nutriment or liability to injury (phylogenetic purely—Darwin, Romanes).
4. Through the struggle of parts for room or for food an unused organ in the individual may be crowded (ontogenetic—Roux). This may lead to the development of the used organ as against the disused through a compensation of growth (Goethe, Saint-Hilaire, Roux); this ontogenetic result becomes phylogenetic through transmission of the acquired character (Roux), or is in its very nature phyloblastic (Kohl).
5. Through the struggle between soma and germ to produce the maximum of efficiency of the former with the minimum expenditure to the latter (ontogenetic and phylogenetic—Lendenfeld).
6. Through germinal selection, the struggle of the representatives of organs in the germ (ontogenetic and phylogenetic—Weismann).
The idea of ontogenetic degeneration is intimately bound up with the idea of phylogenetic degeneration. Logically we ought to consider first the causes of individual degeneration, and then the processes or causes that led to the transmission of this. Practically it is impossible to do so, because many of the explanations are general. Only No. 4 of the above may be taken in the ontogenetic sense purely, though it was certainly also meant to explain phylogenetic degeneration. In many of the explanations of particular cases of degeneration more than one of the above principles are invoked, though only one was meant to be used. In most cases, however, the discussions of degeneration have been in general terms, without direct bearing on any specific instance of degeneration in all its details. It must be evident that such discussions can only by accident lead to right results.
By the Lamarckian ontogenetic degeneration is considered the result of lack of use and consequent diminished blood supply. The results of the diminution caused by the lack of use during one generation are transmitted in some degree to the next generation, which thus starts at a lower level. A continuation of the same conditions leads finally to the great reduction and ultimate disappearance of an organ.
No one, so far as I am aware, has succeeded in accounting for the degeneration of the eye by means of this view. Packard’s[J] explanations are evidently a mixture of Lamarckism and Darwinism.
[J] American Naturalist, September, 1894, vol. xxviii, p. 727.
Packard says: “When a number, few or many, of normal-seeing animals enter a totally dark cave or stream, some may become blind sooner than others,” some having the eye slightly modified by disuse, while others may have in addition physical or functional defects, especially in the optic nerves and ganglia. “The result of the union of such individuals and adaptation to their Stygian life would be broods of young, some with vision unimpaired, others with a tendency to blindness, while in others there would be noticed the first steps in degeneration of nervous power and nervous tissue.” Packard evidently had invertebrates in mind. He clearly admits the cessation of selection or panmixia in that those born with defects may breed with the others. He supposes that the blind fauna may have arisen in but few or several generations, a supposition that may be applicable to invertebrates, but certainly is not to vertebrates. At first those becoming so modified that they can do without the use of their eyes would greatly preponderate over those ‘congenitally blind.’ “So all the while the process of adaptation was going on, the antennæ and other tactile organs increasing in length and in the delicacy of structures, while the eyes were meanwhile diminishing in strength of vision and their nervous force giving out, after a few generations—perhaps only two or three—the number of congenitally blind would increase, and eventually they would, in their turn, preponderate in numbers.” Packard seems here to admit the principle of degeneration as the result of compensation of growth, the nervous force of the eye giving out with the increase of the tactile and olfactory organs. It is somewhat doubtful in what sense the term ‘congenitally blind’ is used, but it probably means born blind as the result of transmitted disuse, rather than blind as the result of fortuitous variation. The effects of disuse are thus supposed, through their transmission, to have given rise to generations of blind animals. The continued degeneration is not discussed.