ps? I regard this argument as capable of further
extension, for wherever in nature we come upon degeneration, it is
taking place by minute steps and with a slowness that makes it not
directly perceptible, and I believe that this in itself justifies us
in concluding that _the same must be true of ascending_ evolution. But
in the latter case the goal can seldom be distinctly recognised while
in cases of degeneration the starting-point of the process can often
be inferred, because several nearly related species may represent
different stages.

In recent years Bateson in particular has championed the idea of
saltatory, or so-called discontinuous evolution, and has collected a
number of cases in which more or less marked variations have suddenly
appeared. These are taken for the most part from among domesticated
animals which have been bred and crossed for a long time, and it is
hardly to be wondered at that their much mixed and much influenced
germ-plasm should, under certain conditions, give rise to remarkable
phenomena, often indeed producing forms which are strongly suggestive
of monstrosities, and which would undoubtedly not survive in free
nature, unprotected by man. I should regard such cases as due to an
intensified germinal selection--though this is to anticipate a
little--and from this point of view it cannot be denied that they have
a special interest. But they seem to me to have no significance as far
as the transformation of species is concerned, if only because of the
extreme rarity of their occurrence.

There are, however, many variations which have appeared in a sudden
and saltatory manner, and some of these Darwin pointed out and
discussed in detail: the copper beech, the weeping trees, the oak with
"fern-like leaves," certain garden-flowers, etc. But none of them have
persisted in free nature, or evolved into permanent types.

On the other hand, wherever enduring types have arisen, we find traces
of a gradual origin by successive stages, even if, at first sight,
their origin may appear to have been sudden. This is the case with
_seasonal Dimorphism_, the first known cases of which exhibited marked
differences between the two generations, the winter and the summer
brood. Take for instance the much discussed and studied form
_Vanessa_ (_Araschnia_) _levana-prorsa_. Here the differences between
the two forms are so great and so apparently disconnected, that one
might almost believe it to be a sudden mutatio