812 
THE PRINCIPLES OF BIOLOGY. 
parthenogenesis and metagenesis in insects, as occurring in 
the Aphides , are similarly accounted for by the fact “ that 
they get plenty of easily assimilated food without exertion. 5 ' 
(2) The remaining portion of the chapter answers the anticipa¬ 
tion set forth at the beginning, and inter alia points out that 
“ the physiological lesson taught us by bees and ants, not 
quite harmonising with the moral lesson they are supposed 
to teach, is that highly-fed idleness is favourable to fertility, 
and that excessive industry has barrenness for its concomitant.” 
Illustrations will be familiar to most of us. # * * 
The last chapter of this series (XI.) commences with the 
inference that, “ considering the difficulties of inductive 
verification, we have (thinks Mr. Spencer) as clear a corres¬ 
pondence between the ci priori and d posteriori conclusions as 
can be expected.” It is pointed out that “ the many factors 
co-operating to the result in every case, are so variable in 
their absolute and relative amounts, that we can rarely disen¬ 
tangle the effect of each one; and have usually to be content 
with qualified inferences.” Though in the mass, organisms 
show a relation between great size and small fertility, yet 
special comparisons among them are to an extent vitiated 
by differences of structure, nutrition, and expenditure. 
But, says Mr. Spencer, “the broad fact is that organisms in 
which the integration and differentiation of matter and 
motion have been carried furthest are those in which the 
rate of multiplication has fallen lowest.” * * * “ How 
is the ratio between Individuation and Genesis established in 
each case?” Many detailed illustrations follow, showing 
that “ all specialities of the reproductive process are due to 
the natural selection of favourable variations, and obviously, 
too, that a survival of the fittest has a share in determining 
the proportion between the amount of matter that goes to 
Individuation and the amount that goes to Genesis.” * * * 
A qualification has to be made. It is pointed out that “ each 
increment of evolution entails a decrement of reproduction 
that is not accurately proportionate, but somewhat less than 
proportionate.” It must always be borne in mind that 
“ each advance in evolution implies an economy.” * * * 
Most interesting illustrations are traced as giving a 
solution of various minor anomalies “ by which the inverse 
variation of Individuation and Genesis is obscured.” A 
comparison between the blackbird and linnet, both of which 
usually lay five eggs and have two broods in a year. Yet the 
blackbird is far the larger of the tw r o, and ought, according to 
the general law, to be much less prolific. Why is it not ? 
The blackbird is omnivorous—the linnet graminivorous. 
