Notices of Books. 
1 877.] 
535 
vigour; and we have a strictly analogous result with our domestic 
animals.” 
The question whether a vegetable species can be reproduced 
asexually, i.e., by rhizomes, stolons, &c., from a very remote 
period remains open. Andrew Knight maintained that a variety 
exclusively thus propagated, like the majority of our fruit trees, 
must ultimately become weakly, and Prof. Asa Gray leans to 
the same view. It would be interesting, if the Anti-ViviseCtion- 
ists would allow it, to take some animal capable of propagation 
by “ cuttings,” and try for how many generations this mode of 
reproduction could be carried on without a visible decay of 
vigour. 
Mr. Darwin guards against the inference that cross-fertilisation 
is, per se, beneficial under all circumstances. His experiments 
show that the “ benefit from cross-fertilisation depends on the 
plants which are crossed having been subjected during previous 
generations to somewhat different conditions.” Thus plants 
which had been self-fertilised for the eight previous generations 
were crossed with plants which had been inter-crossed for the 
same number of generations, all having been kept under the 
same conditions as far as possible ; seedlings from this cross 
were grown in competition with others derived from the same 
self-fertilised mother-plant crossed by a fresh stock, and the latter 
seedlings were to the former in height as 100 : 52, and in fertility 
as 100 : 4.” The advantages of a cross, Mr. Darwin considers, 
“ depend altogether on the differentiation of the sexual elements, 
a conclusion which harmonises perfectly with the faCt that a 
slight and occasional change in the conditions of life is beneficial 
to all plants and animals. We thus see that in many species 
plants fertilised with their own pollen are either absolutely sterile 
or very sparingly fruitful ; if fecundated with pollen from another 
flower on the same plant, they are sometimes a little more fertile ; 
if treated with pollen from another individual or variety of the 
same species, their fertility is at its maximum ; but if with pollen 
from a different species their fertility declines, till w 7 e arrive at 
absolute sterility. “ We have thus a long series with utter ste- 
rility at the two ends ; at one end due to the sexual elements not 
having been sufficiently differentiated, and at the other end to 
their having been differentiated in too great a degree or in some 
peculiar manner.” But having penetrated so far we must confess 
our ignorance. “ We do not know what is the nature or degree 
of the differentiation in the sexual elements which is favourable 
for union, and what is injurious.” Some species are greatly 
benefitted by crossing, while others profit very little. Some 
plants retain their vigour after having been self-fertilised for un- 
told generations. But for these and for many connected faCts 
we can scarcely conjecture a reason. 
Going still further, and admitting — as we are compelled — that 
fertile eggs can be produced without the co-operation of the male, 
