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 in 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. 



