The reproductive organs themselves are not diseased; and the diseases from which animals in menageries usually perish, are not those which in any way affect their fertility.

No domestic animal is more subject to disease than the sheep, yet it is remarkably prolific....

It would appear that any change in {235} the habits of life, whatever these habits may be, if great enough, tends to affect in an inexplicable manner the powers of reproduction." Such, then, is the singular sensitiveness of the generative system.
As to the means by which that system is affected, we see that a variety of conditions affect it; but as to the modes in which they act upon it, we have as yet little if any clue.
We have also seen the singular effects (in tailed Lepidoptera, &c.) of causes connected with geographical distribution, the mode of action of which is as yet quite inexplicable; and we have also seen the innate tendency which there appears to be in certain groups (birds of paradise, &c.) to develop peculiarities of a special kind.
It is, to say the least, probable that other influences exist, terrestrial and cosmical, as yet un-noted.

The gradually accumulating or diversely combining actions of all these on highly sensitive structures, which are themselves possessed of internal responsive powers and tendencies, may well result in occasional repeated productions of forms harmonious and vigorous, and differing from the parental forms in proportion to the result of the combining or conflicting action of all external and internal influences.
If, in the past history of this planet, more causes ever intervened, or intervened more energetically than at present, we might _a priori_ expect a richer and more various evolution of forms more radically differing than any which could be produced under conditions of more perfect equilibrium.
At the same time, if it be true that the last few thousand years have been a period of remarkable and exceptional uniformity as regards this planet's astronomical relations, there are then some grounds for thinking that organic evolution may have been exceptionally depressed during the same epoch.
Now, as to the fact that sudden changes and sudden developments have {236} occurred, and as to the probability that such changes are likely to occur, evidence was given in Chapter IV.
In Chapter V.we also saw that minerals become modified suddenly and considerably by the action of incident forces--as, _e.g._, the production of hexagonal tabular crystals of carbonate of copper by sulphuric acid, and of long rectangular prisms by ammonia, &c.
We have thus a certain antecedent probability that if changes are produced in specific manifestation through incident forces, these changes will be sensible and considerable, not minute and infinitesimal.
Consequently, it is probable that new species have appeared from time to time with comparative suddenness, and that they still continue so to arise if all the conditions necessary for specific evolution now obtain.
This probability will be increased if the observations of Dr.Bastian are confirmed by future investigation.

According to his report, when the requisite conditions were supplied, the transformations which appeared to take place (from very low to higher organisms) were sudden, definite, and complete.
Therefore, if this is so, there must probably exist in higher forms a similar tendency to such change.