3. This truth that a portion of undifferentiated tissue, if adequate in quantity, assumes the structure of the type, illustrating as it does the proclivity of the constitutional units towards the structure of the species, allies itself with the phenomena of both agamogenesis and gamogenesis. The first of these shows us how a fissiparously-detached portion of the parental tissue takes on the same form as the parent; and the second shows how those small detached portions distinguished as sperm-cell and germ-cell also, when united and supplied with the needful materials, do the same thing.
4. But the set of phenomena following the union of sperm-cell and germ-cell differ in a certain way from those which follow when a gemma or other unfertilized portion of parental tissue is detached. The incomprehensibleness of this difference as otherwise contemplated, and the partial comprehensibleness of it when joined with the hypothesis of physiological units, furnish a further support for the hypothesis.
The familiar truth learnt by the tyro in algebra that an apparent solution which contains the unknown quantity is no solution, is a truth apt to be overlooked in other spheres than the algebraic. An illustration is supplied by the answer once given in Parliament to the question “What is an Archdeacon?”—“One who discharges archidiaconal functions.” But science as well as daily life furnishes examples. When it is said by Engelmann, Hensen, Hertwig, and Maupas that “the essential end of sexuality is rejuvenescence, that is, the restoration of growth-energy,” we have another instance of an explanation which explains nothing. What is the phenomenon to be explained? That unfolding of an organism from a germ which displays growth-energy. And what is the explanation? The giving of fresh growth-energy. The unknown quantity “growth-energy” is contained in the explanation proposed. There exists no conception of “juvenescence” save that derived from observing developing plants and animals; and if “re” be prefixed, no interpretation is thereby given to the unexplained thing “juvenescence.”
Coleridge somewhere comments on a source of fallacy which he calls the “hypostasis of a relation”—the changing of a relation into a thing. The plumber who tells you that water rises in a pump “by suction” supplies an instance. Having assumed suction to be an agent, he thinks that he understands how the piston does its work. Some of the explanations given of fertilization supply further instances. When it is said that sexual union has for its end “to give increased vigour to all the vital processes,” it is tacitly implied that vigour is a something—a something which can be given. But now, in the first place, it is only by the hypostasis of a relation that we are led to think of vigour as a thing. Vigour is a state—that state of a living body which enables it to give out much motion. What enables it to do this? The presence in it of abundant molecules containing much molecular motion which can be transformed into molar motion: the transformation being effected by the falling of these molecules into their simpler and relatively-inert components, which are thereupon excreted. Energy-containing matter is used up, and more energy or vigour can be given only by supplying more such matter. How then can the union of two nuclei—those of the sperm-cell and germ-cell—give vigour? Only an infinitesimal portion of vigour in the sense above explained exists in either, and the union of them leaves it still infinitesimal. And then, even supposing the vigour to be an entity and to be appreciable in quantity, how could it go on producing that immense combination of physiological actions seen in the unfolding of the germ into an organism? and how could it go on producing the physiological actions of an adult organism during a whole century?
May we not then say that these proposed explanations leave the question where it was—are nominal solutions, not real solutions?
5. But the hypothesis of constitutional units furnishes, if not a satisfactory answer yet, something in the nature of an answer—a true cause; that is to say, a cause actually known to us as operating in other cases. In [§ 92] it was pointed out that in proportion as units are similar, there may be built up from them an aggregate which is relatively stable, and that along with increasing dissimilarity the stability of the aggregate decreases. It was inferred that if a group of constitutional units belonging to one individual which have become moulded into relatively exact congruity with the organism and with one another by long co-operation, are mingled with some belonging to another individual which, differently circumstanced, has become somewhat different in itself and in its units, then the mass formed by the union of the two groups will be relatively unstable—relatively modifiable by incident forces. Whereas in either organism, no longer perpetually changed in the relations of its parts by growth, there is an approach towards equilibrium between the whole and its components, the components contributed by the two to form a germ, being slightly unlike one another, will not form a group in a state of equilibrium. The group they form will be capable of easy change by incident forces; and they will so be rendered free to follow their proclivities towards the typical form of the species. Inferring this we must also infer that so long as these two sets of slightly different units are not exposed to any constant forces tending to coerce them into the same form, there will continue to exist in the nuclei of all descendant cells this same relative instability and consequent plasticity.
Such evidence as we have verifies this interpretation. There is first the universal fact that development of the germ begins when it is exposed to an incident force—heat—the undulations of which, increasing the oscillations of the mixed units, give them greater freedom to arrange themselves in conformity with their type. We see this alike when spring warmth makes a seed germinate and when the warmth of a sitting hen sets up organization in her eggs. Heat frees the molecules of inorganic matter from local restraints and, as we see in molten metal, lets them yield to other forces; and similarly in this organic matter, the units are made free to follow their proclivities. Then, secondly, there comes the evidence from comparisons between the effects of mixing constitutional units differing in various degrees. Let the cluster of mixed units be derived from animals that are ordinally distinct. Nothing happens. The units each contributes tend to arrange themselves after the parental type. Hence a conflict between the tendencies towards two markedly unlike structures, and no structure arises. Suppose the mixed units come from two kindred species—say horse and ass. The structures which they respectively tend to form, being in their main characters alike, there is such co-operation as produces a working organism but an organism in certain respects imperfect—a mule. Suppose, again, the units come from two varieties of the same species. A perfect organism results, and, as shown by Mr. Darwin when detailing the effects of crossing, an unusually vigorous organism. The units being more unlike than those belonging to the same variety, the instability of the germ-plasm is unusually great, and the transformations which constitute development and action become unusually active. When, as in ordinary cases, the units are supplied by members of the same variety who have not been made very much alike by their antecedents, there follows the usual amount of organic vigour. Coming now to the results of breeding in-and-in—breeding between individuals whose constitutions (i.e. constitutional units) have for generations been growing more alike in the absence of crossing with other stirps—we see that diminution of organic vigour is displayed: there is a decrease in the rate of physiological change. Finally, on coming to a closer relationship, as in marriages between cousins, in whom the constitutional units are more than commonly alike, we see there frequently follows either barrenness or the production of feeble offspring.
All these facts, then, are congruous with the hypothesis that the use of fertilization is the mixing of unlike units, and consequent production of plasticity. Leaving out cases in which the unlikenesses are so great as wholly to prevent co-operation among the units, the degree of vigour, that is, the activity of physiological change, is great where the unlikeness is great and diminishes with the approach towards likeness.