The Principles of Biology, Volume 1 · Herbert Spencer
HEREDITY.
If so, it becomes comprehensible that with the predominance of either group, and the production of the same sex as that of the parent whence it was derived, there will go the repetition not only of the minor sex-traits of that parent but also of any peculiarities he or she possessed, such as monstrosities. Since the two groups are nearly balanced, and since inheritance is never an average of the two parents but a mixture of traits of the one with traits of the other, it is not difficult to see why there should be some irregularity in the transmission of these monstrosities and constitutional tendencies, though they are most frequently transmitted only to those of the same sex.[37] § 84. Unawares in the last paragraph there has been taken for granted the truth of that suggestion concerning Heredity ventured in § 66. Anything like a positive explanation is not to be expected in the present stage of Biology, if at all. We can look for nothing beyond a simplification of the problem; and a reduction of it to the same category with certain other problems which also admit of hypothetical solutions only. If an hypothesis which sundry widespread phenomena have already thrust upon us, can be shown to render the phenomena of Heredity more intelligible than they at present seem, we shall have reason to entertain it. The applicability of any method of interpretation to two different but allied classes of facts, is evidence of its truth.
The power which many animals display of reproducing lost parts, we saw to be inexplicable except on the assumption that the units of which any organism is built have a tendency to arrange themselves into the shape of that organism (§ 65). This power is sufficiently remarkable in cases where a lost limb or tail is replaced, but it is still more remarkable in cases where, as among some annelids, the pieces into which an individual is cut severally complete themselves by developing heads and tails, or in cases like that of the _Holothuria_, which having, when alarmed, ejected its viscera, reproduces them. Such facts compel us to admit that the components of an organism have a proclivity towards a special structure--that the adult organism when mutilated exhibits that same proclivity which is exhibited by the young organism in the course of its normal development. As before said, we may, for want of a better name, figuratively call this power organic polarity: meaning by this phrase nothing more than the observed tendency towards a special arrangement. And such facts as those presented by the fragments of a _Hydra_, and by fragments of leaves from which complete plants are produced, oblige us to recognize this proclivity as existing throughout the tissues in general--nay, in the case of the _Begonia phyllomaniaca_, obliges us to recognize this proclivity as existing in the physiological units contained in each undifferentiated cell. Quite in harmony with this conclusion, are certain implications since noticed, respecting the characters of sperm-cells and germ-cells. We saw sundry reasons for rejecting the supposition that these are highly-specialized cells and for accepting the opposite supposition, that they are cells differing from others rather in being unspecialized. And here the assumption to which we seem driven by the _ensemble_ of the evidence, is, that sperm-cells and germ-cells are essentially nothing more than vehicles in which are contained small groups of the physiological units in a fit state for obeying their proclivity towards the structural arrangement of the species they belong to.
If the likeness of offspring to parents is thus determined, it becomes manifest, _à priori_, that besides the transmission of generic and specific peculiarities, there will be a transmission of those individual peculiarities which, arising without assignable causes, are classed as "spontaneous." For if the assumption of a special arrangement of parts by an organism, is due to the proclivity of its physiological units towards that arrangement; then the assumption of an arrangement of parts slightly different from that of the species, implies physiological units slightly unlike those of the species; and these slightly-unlike physiological units, communicated through the medium of sperm-cell or germ-cell, will tend, in the offspring, to build themselves into a structure similarly diverging from the average of the species. But it is not equally manifest that, on this hypothesis, alterations of structure caused by alterations of function must be transmitted to offspring. It is not obvious that change in the form of a part, caused by changed action, involves such change in the physiological units throughout the organism that these, when groups of them are thrown off in the shape of reproductive centres, will unfold into organisms that have this part similarly changed in form. Indeed, when treating of Adaptation (§ 69), we saw that an organ modified by increase or decrease of function, can but slowly re-act on the system at large, so as to bring about those correlative changes required to produce a new equilibrium; and yet only when such new equilibrium has been established, can we expect it to be _fully_ expressed in the modified physiological units of which the organism is built--only then can we count on a complete transfer of the modification to descendants. Nevertheless, that changes of structure caused by changes of action must also be transmitted, however obscurely, appears to be a deduction from first principles--or if not a specific deduction, still, a general implication.
For if an organism A, has, by any peculiar habit or condition of life, been modified into the form A', it follows that all the functions of A', reproductive function included, must be in some degree different from the functions of A. An organism being a combination of rhythmically-acting parts in moving equilibrium, the action and structure of any one part cannot be altered without causing alterations of action and structure in all the rest; just as no member of the Solar System could be modified in motion or mass, without producing rearrangements throughout the whole Solar System. And if the organism A, when changed to A', must be changed in all its functions; then the offspring of A' cannot be the same as they would have been had it retained the form A. That the change in the offspring must, other things equal, be in the same direction as the change in the parent, appears implied by the fact that the change propagated throughout the parental system is a change towards a new state of equilibrium--a change tending to bring the actions of all organs, reproductive included, into harmony with these new actions.
Or, bringing the question to its ultimate and simplest form, we may say that as, on the one hand, physiological units will, because of their special polarities, build themselves into an organism of a special structure; so, on the other hand, if the structure of this organism is modified by modified function, it will impress some corresponding modification on the structures and polarities of its units. The units and the aggregate must act and re-act on each other. If nothing prevents, the units will mould the aggregate into a form in equilibrium with their pre-existing polarities. If, contrariwise, the aggregate is made by incident actions to take a new form, its forces must tend to re-mould the units into harmony with this new form. And to say that the physiological units are in any degree so re-moulded as to bring their polar forces towards equilibrium with the forces of the modified aggregate, is to say that when separated in the shape of reproductive centres, these units will tend to build themselves up into an aggregate modified in the same direction. NOTE.--A large amount of additional evidence supporting the belief that functionally produced modifications are inherited, will be found in Appendix B.
Text from Project Gutenberg, public domain in the United States.