PRINCIPLES OF BIOLOGY. 
43 
entire individuals, as in the Monads, and the fusion of their 
contents as in the Diatoms and Desmids, and in the Conju¬ 
gate Algie (for there each cell is practically a distinct indi¬ 
vidual) upwards to the higher animals and plants, in which 
the portion separated for the reproductive process is but an 
extremely small part of the parent organism. 
The mysterious result of this union suggests that the cells 
which take part in it are specialised in some peculiar way, 
but the evidence goes to prove that they are rather un- 
specialised; that, in fact, in proportion as cells are specialised 
they are unfitted for reproductive purposes. Accepting 
Herbert Spencer’s hypothesis of physiological units, we may 
say that a cell which is fitted to reproduce the species must 
contain all the physiological units essential for that species, 
and that specialisation consists in the removal of certain of 
these units, so that some kinds of them either entirely 
disappear or are reduced below the necessary standard. 
The fact that the reproductive cells, in most cases, are 
capable of only a very slight further growth, if they remain 
un-united with each other, is a proof that the units of which 
they are composed are very nearly in a state of stable equili¬ 
brium among themselves ; by their mixture, the equilibrium is 
destroyed and a new series of structural changes is instituted. 
Why does this mixture occur, and when.? Here we 
consider only the latter question, reserving the attempt 
to supply an answer to the former for a future chapter. 
It is found that in most cases in the higher plants agamo- 
genesis prevails when nutrition is abundant, and that when 
from any cause nutrition becomes reduced nearly to the level 
of expenditure, that is, when active growth is beginning to 
cease, then gamogenesis intervenes. The same connection is 
observable in many animals. I need not recount these 
well-known instances, but it is interesting to notice that 
illustrations of the same law are more and more being 
observed among the lower plants. It has long been known 
that to obtain the zygospores of Mucor it is necessary to 
starve the plant, to grow it without access of air; and 
Brefeld’s more recent discoveries prove that the same method 
is required to obtain the sexual (sclerotoid) condition of 
Penicillium, which in the natural state is probably very rare. 
In fact, the instances of the law thus connecting innutrition 
and reproduction are so numerous, and the exceptions 
comparatively so few, though well marked, that it cannot but 
be true as a general rule, though probably complicated with 
some other law (unknown) which in certain cases interferes 
with its action. 
