•_' Proceedings of the Royal Physical Society. 



I include those parts of the subject which are the special domain of the 

 systeraatist, 1 and regarding which all deference must be paid to his special 

 knowledge. The opinion of the systematist is of the greatest possible weight 

 in regard to the relatively minute and often obscure differences which go, 

 some of them, to constitute generic and specific differences, others to form 

 individual variations. 



It will probably be generally admitted that instability — whether it finds 

 its expression in the ceaseless processes of change which are grouped together 

 under the term Metabolism, or in the processes which make every living 

 individual differ to some extent from every other living organism — is an 

 inherent characteristic of living substance; is, indeed, one of those qualities 

 which go together to make up what we call Life. If this be admitted, it 

 is clear that this instability must form the basis of evolutionary change, and 

 that the actual causes of Evolution are to be sought for amongst factors which 

 will favour the departures from the " normal " occurring in certain definite 

 directions, and will thus bring about the relative concentration of variations 

 in these particular directions, so that by their aggregation a process of 

 evolutionary change will be brought about. 



Variability Essential to Survival. 



It should be accentuated that mere variability — i.e., the tendency to 

 vary indiscriminately in different directions — is of the first importance 

 to the survival of strains of organisms. Nothing probably is more certainly 

 fatal to a species than the loss of its variability. A species proceeding 

 along the evolutionary path is as certainly doomed by the loss of its 

 variability as is a moving bicycle with its front wheel clamped so that 

 it can not deviate either to the right or to the left. If we could study 

 the environment of a strain of organisms through a prolonged period of 

 evolutionary time, we should find that the environmental conditions show 

 constant slight fluctuations, some of them consisting of climatic or other 

 physical changes, others of changes in organic environment. To each of 

 these the ever mobile species adapts itself, and upon the faculty of so doing 

 its continued existence depends. 



1 I use the word in its ordinary sense. Strictly speaking the systematist is a niorpho- 

 logist, and the morphologist is a systematist. Both have as one of their main objects the 

 working out of a perfect genetic classification of the animal kingdom. The different 

 methods of investigation necessitated by the more deeply-seated features characteristic 

 "i ihe in. mi group; un ih,' one hand, and 03 the more superficial and often minute features 

 characteristic of tin- subsidiary groups such as genera and species on the other, has not 

 111111.it 111. illy resulted in practice into the division of workers along these lines into two 

 si-paratt: groups. 



