There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has lately
been insisted on by Professor Cope and others in the United States. It is
now known that some animals are capable of reproduction at a very early
age, before they have acquired their perfect characters; and if this power
became thoroughly well developed in a species, it seems probable that the
adult stage of development would sooner or later be lost; and in this case,
especially if the larva differed much from the mature form, the character
of the species would be greatly changed and degraded. Again, not a few
animals, after arriving at maturity, go on changing in character during
nearly their whole lives. With mammals, for instance, the form of the
skull is often much altered with age, of which Dr. Murie has given some
striking instances with seals. Every one knows how the horns of stags
become more and more branched, and the plumes of some birds become more
finely developed, as they grow older. Professor Cope states that the teeth
of certain lizards change much in shape with advancing years. With
crustaceans not only many trivial, but some important parts assume a new
character, as recorded by Fritz Muller, after maturity. In all such cases-
-and many could be given--if the age for reproduction were retarded, the
character of the species, at least in its adult state, would be modified;
nor is it improbable that the previous and earlier stages of development
would in some cases be hurried through and finally lost. Whether species
have often or ever been modified through this comparatively sudden mode of
transition, I can form no opinion; but if this has occurred, it is probable
that the differences between the young and the mature, and between the
mature and the old, were primordially acquired by graduated steps.

SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION.

Although we must be extremely cautious in concluding that any organ could