That tendency may indeed be long suppressed, and ultimately modified by the action of heredity--an action which would increase in force with the increase in the perfection and complexity of the organism affected.

Still we might expect that such changes as do take place would be also sudden, definite, and complete.
Moreover, as the same causes produce the same effects, several individual parent forms must often have been similarly and simultaneously affected.
That they should be so affected--at least that several similarly modified individuals should simultaneously arise--has been seen to be a generally necessary circumstance for the permanent duration of such new modifications.
It is also conceivable that such new forms may be endowed with {237} excessive constitutional strength and viability, and with generative prepotency, as was the case with the black-shouldered peacock in Sir J.
Trevelyan's flock.

This flock was entirely composed of the common kind, and yet the new form rapidly developed itself "_to the extinction of the previously existing breed_."[242] Indeed, the notion accepted by both Mr.Darwin and Mr.Herbert Spencer, and which is plainly the fact (namely, that changes of conditions and incident forces, within limits, augment the viability and fertility of individuals), harmonizes well with the suggested possibility as to an augmented viability and prepotency in new organic forms evolved by peculiar consentaneous actions of conditions and forces, both external and internal.
The remarkable series of changes noted by Dr.Bastian were certainly not produced by external incident forces _only_, but by these acting on a peculiar _materia_, having special properties and powers.

Therefore, the changes were induced by the consentaneous action of internal and external forces.[243] In the same way then, we may expect changes in higher forms to be evolved by similar united action of internal and external forces.
One other point may here be alluded to.

When the remarkable way in which structure and function simultaneously change, is borne in mind; when those numerous instances in which nature has supplied similar wants by similar means, as detailed in Chapter III., are remembered; when also all the wonderful contrivances of orchids, of mimicry, and the strange complexity of certain instinctive actions are considered: then the conviction forces itself on many minds that the organic world is the expression of an intelligence of some kind.