In each of the organic sub-kingdoms the change from an incoherent, indefinite homogeneity to a coherent definite heterogeneity is illustrated in a quadruple way. The originally-like units or cells become unlike, in various ways, and in ways more numerously marked as the development goes on. The several tissues which these several classes or cells form by aggregation, grow little by little distinct from each other; and little by little become structurally complex. In the shoot as in the limb, the external form, originally very simple and having much in common with countless simple forms, organic and inorganic, gradually acquires an increasing complexity, and an increasing unlikeness to other forms, and meanwhile, the remaining parts of the organism, having been developed severally, assuming structures diverging from each other and from that of this particular shoot or limb, there has arisen a greater heterogeneity in the organism as a whole. The most remarkable induction of von Baer comes next in order. It is that in its earliest stage every organism has the greatest number of characters in common with all other organisms in their earliest stages; that at each subsequent stage traits are acquired which successively distinguish the developing embryo from groups of embryos that it previously resembled--thus step by step diminishing the group of embryos which it still resembles; and that thus the class of similar forms is finally narrowed to the species of which it is a member. For example, the human germ, primarily similar to all others, first differentiates from vegetal germs, then from invertebrate germs, and subsequently assumes the mammalian, placental unguiculate, and lastly the human characters. The development of an individual organism is at the same time a differentiation of its parts from each other and a differentiation of the consolidated whole from the environment; and in the last as in the first respect there is a general analogy between the progression of an individual organism and the progression of the lowest orders of organisms to the highest orders. _The Laws of Multiplication_ Every living aggregate being one of which the inner actions are adjusted to balance outer actions, it follows that the maintenance of its moving equilibrium depends on its exposure to the right amounts of these actions. Its moving equilibrium may be overturned if one of these actions is either too great or too small in amount: either by excess or defect of some inorganic or organic agency in its environment. Our inquiry resolves itself into this:--in races that continue to exist what laws of numerical variation result from these variable conflicting forces? The forces preservative of a race are two--ability in each member of the race to preserve itself, and ability to produce other members. These must vary inversely--one must decrease as the other increases. We have to ask in what way this adjustment comes about as a result of evolution.