Still stronger becomes the probability of this view when we call to mind the fact, that the same germ may be evolved into different forms according to circumstances.  Thus, during its earlier stages, every embryo is sexless—­becomes either male or female as the balance of forces acting upon it determines.  Again, it is a well-established fact that the larva of a working-bee will develop into a queen-bee, if, before it is too late, its food be changed to that on which the larvae of queen-bees are fed.  Even more remarkable is the case of certain entozoa.  The ovum of a tape-worm, getting into its natural habitat, the intestine, unfolds into the well-known form of its parent; but if carried, as it frequently is, into other parts of the system, it becomes a sac-like creature, called by naturalists the Echinococcus—­a creature so extremely different from the tape-worm in aspect and structure, that only after careful investigations has it been proved to have the same origin.  All which instances imply that each advance in embryonic complication results from the action of incident forces upon the complication previously existing.

Indeed, we may find a priori reason to think that the evolution proceeds after this manner.  For since it is now known that no germ, animal or vegetable, contains the slightest rudiment, trace, or indication of the future organism—­now that the microscope has shown us that the first process set up in every fertilised germ, is a process of repeated spontaneous fissions ending in the production of a mass of cells, not one of which exhibits any special character:  there seems no alternative but to suppose that the partial organisation at any moment subsisting in a growing embryo, is transformed by the agencies acting upon it into the succeeding phase of organisation, and this into the next, until, through ever-increasing complexities, the ultimate form is reached.  Thus, though the subtilty of the forces and the slowness of the results, prevent us from directly showing that the stages of increasing heterogeneity through which every embryo passes, severally arise from the production of many changes by one force, yet, indirectly, we have strong evidence that they do so.

We have marked how multitudinous are the effects which one cause may generate in an adult organism; that a like multiplication of effects must happen in the unfolding organism, we have observed in sundry illustrative cases; further, it has been pointed out that the ability which like germs have to originate unlike forms, implies that the successive transformations result from the new changes superinduced on previous changes; and we have seen that structureless as every germ originally is, the development of an organism out of it is otherwise incomprehensible.  Not indeed that we can thus really explain the production of any plant or animal.  We are still in the dark respecting those mysterious properties in virtue of which the germ, when subject to fit influences, undergoes the special changes that begin the series of transformations.  All we aim to show, is, that given a germ possessing these mysterious properties, the evolution of an organism from it, probably depends upon that multiplication of effects which we have seen to be the cause of progress in general, so far as we have yet traced it.