als, which have already ceased to yield seeds; so that this acme
of sterility, when the germen alone is effected, cannot have been gained
through selection; and from the laws governing the various grades of
sterility being so uniform throughout the animal and vegetable kingdoms,
we may infer that the cause, whatever it may be, is the same or nearly
the same in all cases.

We will now look a little closer at the probable nature of the
differences between species which induce sterility in first crosses and
in hybrids. In the case of first crosses, the greater or less difficulty
in effecting a union and in obtaining offspring apparently depends
on several distinct causes. There must sometimes be a physical
impossibility in the male element reaching the ovule, as would be the
case with a plant having a pistil too long for the pollen-tubes to
reach the ovarium. It has also been observed that when the pollen of one
species is placed on the stigma of a distantly allied species, though
the pollen-tubes protrude, they do not penetrate the stigmatic surface.
Again, the male element may reach the female element, but be incapable
of causing an embryo to be developed, as seems to have been the case
with some of Thuret's experiments on Fuci. No explanation can be given
of these facts, any more than why certain trees cannot be grafted on
others. Lastly, an embryo may be developed, and then perish at an early
period. This latter alternative has not been sufficiently attended to;
but I believe, from observations communicated to me by Mr. Hewitt, who
has had great experience in hybridising pheasants and fowls, that the
early death of the embryo is a very frequent cause of sterility in first
crosses. Mr. Salter has recently given the results of an examination of
about 500 eggs produced from various crosses between three species
of Gallus and their hybrids; the majority of these eggs had been
fertilised; and in the majority of the fertilised eggs, the embryos had
either been partially developed and had then perished, or had become
nearly mature, but the young chickens had been unable to break through
the shell. Of the chickens which were born, more than four-fifths died
within the first few days, or at latest weeks, "without any obvious
cause, apparently from mere inability to live;" so that from the 500
eggs only twelve chickens were reared. With plants, hybridized embryos
probably often perish in a like manner; at least it is known that