1899] THE SCOPE OF NATURAL SELECTION 189 
animals choosing different foods, the same blood circulating in the 
body of one animal yet has different substances extracted from it by 
different tissues; wherever we look we see life display this selective 
action towards its environment; if the materials that supply its needs 
are not present, the organism dies. This constant and universal 
tendency in living tissue to select out of many substances its own 
particular foods is not favourable to any theory of direct climatic 
modification ; it does, however, favour the principle of selective 
adaptation. 
The phenomena grouped around reproduction, in so far as it 
consists 1n conjugation and sex differentiations, seem to me to be 
explainable only on the assumption that protoplasm is scarcely, if at 
all, climatically modifiable. The simplest form of reproduction is that 
of simple fission; the single celled organism in which it occurs splits 
into two or more divisions. Spencer has suggested that the reason for 
this division may be, that unless very exceptional conditions of growth 
arise, there will be a constant tendency for volume to increase relatively 
to surface, and consequently that a point would at last be reached 
when certain portions of the cell would be insufficiently nourished. 
To decrease bulk and increase surface division would be necessary ; 
such a theory of fission formed on mechanical grounds offers no 
difficulty to selection or other theories. 
But if the relation that bulk bears to surface determines fission, 
it follows that fission will be favoured, as we have seen, by poor food- 
supply and by rapid metabolism, while the opposite conditions will 
favour slow metabolism; under the first set of conditions a small 
rapidly dividing cell would be favoured, while conditions that 
favoured slow metabolism would produce a large cell. On any 
system of climatic inheritance, the structure and needs of the 
organism would be modified according to the environment, hence one 
can see no need for conjugation. On any hypothesis that relies 
mainly or wholly on selection, it is, on the contrary, easy to under- 
stand that union of two nearly allied individuals would tend to 
preserve the stability in so far as they were allied, and would 
promote variability on the unallied smaller portion; there would 
be as a result an increased number of possible variations to select 
from, and those organisms in which conjugation occurred would 
be more likely to survive under all conditions, as they would always 
tend to adapt more readily. A certain limited unlikeness in 
the two cells which entered into combination would be favoured by 
natural selection, in order to preserve this necessary variability. This 
unlikeness might be the beginning of sex differentiation. The fact 
that conjugation occurs at all, may be explained in part by the fact 
that all living tissue has a certain selective affinity (and in this it 
presents many analogies to non-living) for what it has need of; 
conjugation might be merely the satisfaction of an organismic need. 
