Page:Popular Science Monthly Volume 8.djvu/741

Rh principle. Sound philosophy should discredit all such fanciful ideas. The tendency merely signifies that these polarities, being complexes of the physiological units, can find equilibrium only in the form of the adult organism to which they belong. To this equilibrium they tend, not only by an internal impulsion, but also under the combined action of external forces: the latter represent the force which arranges the units in a new order, and the former the direction in which this force is exerted. Now, the cells which go to reproduce an organism are in a state of unstable equilibrium and of minimum heterogeneity: but they are not indifferent substances; they are the vehicles of physiological units derived from the parents, and they follow only the tendency impressed upon them by their polarities. The same is to be said of the elements of the plasma from which a tissue or an organ is reproduced. Thus we see that the resemblance of an organism to the organisms from which it is sprung is the result of the tendencies proper to the physiological units which have come from the parents.

In the fecundated germ there are two groups of physiological units, presenting in their structures slight differences, so that by their fundamental resemblance they conspire to form an organism of the species to which the parents belong, and by their differences they give to this organism traits peculiar to each of the two parents. In this way, simultaneously with transmission of generic and specific characters, we have transmission of those which are peculiar to the individual. Further, we see that characters due to variations called accidental or spontaneous, because we are unable to assign their true cause, must also be transmitted as a tendency of the physiological units, provided this character has gained in the individual such a degree of stability as henceforth to find its place in that individual's state of equilibrium. The action of the surrounding conditions will determine whether the tendency of the physiological units is to be realized or frustrated. The tendency of the physiological units expresses an internal equilibrium, and hence heredity is a consequence of our first principles.

One character of living things is the faculty of reproducing themselves, i. e., of emitting parts of themselves which develop into perfect individuals. This property, in all respects analogous to that which reproduces tissues, differs from the latter only as regards the production of new individuals, or only parts of the same individual. There is an analogy between the operation of generation and that of repair, but there is also a difference. In repair the new products are aggregated around the same axis as the old, whereas in generation the new product soon becomes itself the axis around which the increments of nutrition group themselves. In reality, the contrasts are in excess of the analogies; generation is at bottom an operation of disintegration. This is very well seen in those low organisms which produce