174 KEPORT— 1891. 



influence of Darwin's writings that in this respect a very im- 

 portant change has come over biological research — a change 

 which has been penetrating further and further year after year, 

 till now it may be said that there is hardly any part of the 

 great domain of biology that it has not reached. This change 

 has been, in great measure, in the nature of an illumination, 

 and the illuminating iiafluence has been theory, and more es- 

 pecially the theories of descent and modification by natural 

 selection. And this illuminating influence, which has lent 

 tenfold interest to the work of every investigator of animated 

 nature, has also shown to him many new lines of study, in 

 following up which he is conscious that, while not leaving his 

 particular corner of the field, he is doing work that is of interest 

 to a comparatively wide circle of thinking men. 



The field of biology is so wide, and there are so many pro- 

 blems awaiting solution, that there is a corresponding variety in 

 the subject-matter of the biological tl^eories constantly being 

 put forward. But most of them may, I think, be compre- 

 hended in the follovv'ing main groups : — 



1. Theories referring to the ultimate constitution of organ- 

 ized bodies, usually seeking to explain various special properties 

 of living matter — assimilation, growth, reproduction, heredity 

 — by something in the form and mode of grouping of the 

 ultimate elements. 



2. Theories aiming at an explanation of the mechanism by 

 which the living world has become evolved, dealing with the 

 shares taken respectively by natural selection, the action of 

 external conditions, and other influences in the process of 

 evolution. 



3. Theories which have to do more particularly with the 

 genealogy of the animal and vegetable kingdoms as a whole, 

 or with that of special groups — in other words, with the rela- 

 tionship of living things — with which must be closely con- 

 nected theories of the homologies of organs. 



4. Theories which, assuming the truth of some general 

 theory of evolution, seek to explain individual phenomena in 

 terms of this general theory. 



There are many theories that do not come very well under 

 any of the above headings, but these will comprehend at least 

 the main body ; and, without attempting to deal with the 

 whole subject comprehensively, I shall direct your attention 

 to recent theories belonging to each of these groups. 



As regards the last group, they, for the most part, take 

 natural selection for granted, and aim at showing that some 

 structure is useful to the organism, or is the remnant of a 

 formerly useful part. 



Instances of this variety of theory are very numerous. Of 



