Inheritance
The remarkable facts of inheritance, extending to the reproduction of unimportant peculiarities of parts or organs (rudimentary parts) mentioned above, and the occasional outbreak of ancestral characters that have been dormant through several generations (some of which I will mention further on), might be thought perfectly unexplainable; but they are readily accounted for by the supposition that each part of an organism contributes its constituent and effective molecules to the germ and sperm particles. Mr. Sorby made numerous investigations with relation to the number of molecules in the germinal matter of eggs, and the spermatic matter supplied by the male. Omitting the alkali, Mr. Sorby takes the formula, C72H112N18SO22, as representing the composition of albumen. In a 1⁄2000 of an inch cube, he reckons—
| Albumen | 18,000,000,000,000 | molecules. | |
| Water | 992,000,000,000,000 | ″ | |
| ————————————— | |||
| 1,010,000,000,000,000 | molecules. | ||
Or, in a sphere of the same diameter, 530,000,000,000,000 of the two components. Taking a single mammalian spermatozoon, having a mean diameter of 1⁄6000 of an inch; “it might contain two and a half million of such gemmules. If these were lost, destroyed, or fully developed at the rate of one in each second, this number would be exhausted in about one month; but since a number of spermatozoa appears to be necessary to produce perfect fertilization, it is quite easy to understand that the number of gemmules introduced into the ovum may be so great that the influence of the male parent may be very marked, even after having been, as regards particular character, apparently dormant for many years.” The germinal vesicle of a mammalian ovum being about 1⁄1000 of an inch, mean diameter, might contain five hundred million of gemmules, which, if used up at the rate of one per second, would last more than seventeen years. If the whole ovum, about 1⁄150 in diameter, were all gemmules, the number would be sufficient to last, at this rate, one per second for 5,600 years! This, however, is not probable; but Mr. Sorby’s remarks has completely removed all doubt as to its physical possibility from the Darwinian theory; “and they prompt us,” says Slack, “to a wonderful conception of the powers residing in minute quantities of matter.”
The laws of inheritance are divisible into two series, conservative and progressive transmission; the laws of adaptation to direct (active) or indirect (potential) adaptation.
External causes often influence the reproductive system, especially in organism propagating in a sexual way. This can be strikingly shown in artificially produced monstrosities. Monstrosities can be produced by subjecting the parental organism to certain extraordinary conditions of life; and curiously enough, such an extraordinary condition of life does not produce a change of the organism itself, but a change in its descendants. The new formation exists in the parental organism only as a possibility (potential); in the descendants it becomes a reality (actual). Most commonly, monstrosities with very abnormal forms are sterile, but there are instances where they reproduce their kind and become a species.35 Geoffroy St. Hilaire, who perhaps made the deepest investigations ever conducted into the nature and causes of their production, first conceived the idea of artificially producing them, and to this end he began modifications of the physical conditions of the evolution of the chicken during natural and artificial incubation. He determined the fact that monsters could be produced in this way, but scarcely carried his investigation further. This work has been taken up by M. Dareste, and he has lately published a volume in Paris which recounts the results of a quarter of a century’s experimenting. Eggs, he states, were submitted to incubation in a vertical instead of a horizontal position; they were covered with varnish in certain places so as to stop or modify evaporation and respiration. The evolution of the chick was rendered slower by a temperature below that of the normal heat of incubation. Finally, eggs were warmed only at one point, so that the young animal, during development, was submitted at different parts to variable temperatures.
These perturbations resulted in the most curious and unlooked for deformities in the embryo, some being not alone peculiar to the bird, but being similar to those which have been recognized in many other animals, and even in the human species. The data obtained have been deemed so important that M. Dareste has recently received the Lacaze prize for physiology from the French Academy of Sciences.
It would be impossible to review even a fraction of the many forms of monstrosities which M. Dareste has discovered. Those that we give will, however, suffice to convey an idea of the wonderful variations produced. Fig. 1 is a chick embryo with the encephalon entirely outside the head, the heart, liver, and gizzard outside the umbilical opening, right wing lifted up beside the head, and the development of the left one stopped. In Fig. 2 the encephalon is herniated and marked with blood spots, the eye is rudimentary and replaced by a spot of pigment, the upper beak is shorter than the lower one, while the heart, liver, etc., are all outside. In Figs. 3 and 4 the head is compressed, eyes well developed, but in the back instead of in the sides of the head; the body is bent, abdominal intestines not closed, heart largely developed and herniated. The literal references to the foregoing are: am, amnion; al, allantois; v, vitellus; h, encephalon; i, eye; c, heart; f, liver; g, gizzard; ms, upper, and mi, lower member.
The commonest case of monstrosity observed by M. Dareste has been that of the head protruding from the navel, and the heart or hearts above the head. This is a most extraordinary and new monster, and, if it persist, a chicken with its heart on its back, like a hump, may be expected. A curious fact discovered is the duplicity of the heart at the beginning of incubation, two hearts, beating separately, being clearly seen. Another anomaly consists in heads with a frontal swelling, which is filled by the cerebral hemispheres.
M. Dareste’s artificial monsters are all produced from the single germ or cicatricule (as the white circular spot seen in the yellow of the egg, and from which the embryo springs, is termed). He has not yet been able to determine artificially the production of monsters, the origin of which takes place in a peculiar state of the cicatricule before incubation. But having submitted to incubation some 10,000 eggs, he has obtained several remarkable examples of double monstrosities in process of formation, some representations of which are given herewith. Fig. 5 shows three embryos, all derived from a single cicatricule. Fig. 6 represents three embryos from two cicatricules. On one side of the line of junction are two imperfectly developed embryos, one having no heart. The single embryo on the other side is generally normal, but has a heart on the right side. In Fig. 7 are twins, one well formed, the heart circulating colorless blood, the other having no heart and a rudimentary head. Fig. 8 exhibits a double monster with lateral union. The heads are separate, and there are three upper and three lower members, those of the latter on the median line belonging equally to each of the pair.