at 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. 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