By way of preparation observe how numerous are the changes which any markedstimulus works on an adult organism -- a human being for instance. An alarmingsound or sight, besides impressions on the organs of sense and the nerves,may produce a start, a scream, a distortion of the face, a trembling consequenton general muscular relaxation, a burst of perspiration, and perhaps an arrestof the heart followed by syncope; and if the system be feeble, an illnesswith its long train of complicated symptom may set in. Similarly in casesof disease. A minute portion of the small-pox virus taken into the systemwill, in a severe case, cause, during the first stage, rigors, heat of skin,accelerated pulse, furred tongue, loss of appetite, thirst, epigastric uneasiness,vomiting, headache, pats in the back and limbs, muscular weakness, convulsions,delirium, etc.; in the second stage, cutaneous eruption, itching, tingling,sore throat, swelled fauces, salivation, cough, hoarseness, dyspnoea, etc.;and in the third stage, oedematous inflammations, pneumonia, pleurisy, diarrhoea,inflammation of the brain, ophthalmia, erysipelas, etc.: each of which enumeratedsymptoms is itself more or less complex. Now it needs only to consider thatthis working of many changes by one force on an adult organism, must be partiallyparalleled in an embryo-organism, to understand that in it too there mustbe a multiplication of effects, ever tending to produce increasing heterogeneity.
Each organ as it is developed, serves, by its actions and reactions on therest, to initiate new complexities. The first pulsations of the foetal heartmust simultaneously aid the unfolding of every part. The growth of each tissue,by taking from the blood special proportions of elements, must modify theconstitution of the blood; and so must modify the nutrition of all the othertissues. The distributive actions, implying as they do a certain waste, necessitatean addition to the blood of effete matters, which must influence the restof the system, and perhaps, as some think, initiate the formation of excretoryorgans. The nervous connexions established among the viscera must furthermultiply their mutual influences. And so is it with every modification ofstructure -- every additional part and every alteration in the ratios ofparts. Proof of a more direct kind is furnished by the fact, that the samegerm may be evolved into different forms according to circumstances. Thus,during its earliest stages, every germ is sexless -- originates either maleor female as the balance of forces acting on it determines. Again, thereis the familiar truth that the larva of a working-bee will develop into aqueen-bee if, before a certain period, it is fed after a manner like thatin which the larvae of queen-bees are fed. Then there is the still more strikingevidence furnished by ants and termites. Riley, Grassi, Haviland, and Hart,have shown that differences of nutrition not only originate the differencesbetween males and females but also the different traits of solders, workers,and nurses.*<* See Principles of Biology, Vol. I, pp. 680-8.> Varyingdegree of nutrition, after initiating the unlikeness of sex, then determinesthe unlikenesses of external organs possessed by the various classes of sexlessindividuals. Next comes the evidence, still more directly relevant, suppliedby the effects of castration. If the removal of certain organs prevents thedevelopment of certain other organs in remote parts of the system -- in manthe vocal structures, the beard, some traits of general form, some instinctsand other mental characters -- then it is clear that where these organs havenot been removed, the presence of them determines the occurrence of thesevarious changes of development, and doubtless many minor ones which are unobtrusive.
Here the fact that one cause produces many effects in the course of organicevolution is indisputable. Doubtless we are, and must ever continue, unableto conceive those mysterious properties which make the germ when subjectto fit influences undergo the special changes initiating, and mainly constituting,the transformations of an unfolding organism; though we may consistentlysuppose that they represent an infinite series of inherited modificationsconsequent on the instability of the homogeneous, the multiplication of effects,and one further factor still to be set forth. All here contended is that,given a germ possessing these mysterious properties, the evolution of anorganism from it depends, in part, on that multiplication of effects whichwe have seen to be one cause of evolution in general, so far as we have yettraced it.
When, leaving the development of single plants and animals, we pass tothat of the Earth's Flora and Fauna, the course of the argument again becomesclear and simple. Though, as before admitted, the fragmentary facts Palaeontologyhas accumulated, do not clearly warrant us in saying that, in the lapse ofgeologic time, there have been evolved more heterogeneous organisms, andmore heterogeneous assemblages of organisms; yet we shall now see that theremust ever have been a tendency towards these results. We shall find thatthe production of many effects by one cause, which, as already shown, hasbeen all along increasing the physical heterogeneity of the Earth, has furthernecessitated an increasing heterogeneity of its inhabiting organisms, individuallyand collectively. An illustration will make this clear.
