SECTION D.—ZOOLOGY. 
ORGANIC EVOLUTION. 
ADDRESS BY 
C. TATE REGAN, F.R.S., 
PRESIDENT OF THE SECTION. 
A sysTreMAtic zoologist, whose work is the classification of animals, 
should so define his groups that another worker may be able to use his 
system to place an animal successively in its right class, order, family, 
genus and species, and so arrive at its correct name. The name is the 
key to all that has been recorded about the structure, variation, habits 
and life-history of that particular form; but it should be something more, 
it should be an indication of its relationships ; for it may often happen 
that very little is known about a species, but much about its nearest 
allies. It is, therefore, of practical importance that classification should 
be natural, an expression of relationships; to make it so the systematist 
has to attempt to estimate the meaning of resemblances and differences, 
to what extent they may be due to the nearness or remoteness of a common 
ancestor, to what extent to other circumstances. 
Every good systematist must feel some satisfaction when he has written 
a diagnosis that is diagnostic, or has made a key that will work; but this 
satisfaction is small in comparison with that which he feels when he has 
reason to think he has settled the position of some doubtful form, or has 
discovered the origin of a group and the lines of evolution within it, or 
_ has found the relation between structure and habits or environment. The 
main interest of systematic work lies in the fact that it is a study of the - 
results of evolution, and that from such a study one may hope to get some 
light on the meaning of evolution. 
For any profitable discussion of the origin of species it is essential 
to know what we mean when we use the word ‘species.’ In nature we 
find that a number of similar individuals, with similar habits, live in a 
certain area; such an aggregation of individuals may be termed a com- 
munity. It is unfortunate that this word has sometimes been used for 
‘dissimilar and unrelated organisms that occur together—for example, the 
animals found on a muddy bottom in the North Sea, or the plants of a 
range of chalk hills; but I am satisfied that the word ‘ association’ is more 
‘appropriate to these, and that ‘ community ’ is the right name for a number 
‘of similar individuals that live together and breed together. All this is 
preliminary to my definition of a species. A species is a community, 
or a number of related communities, whose distinctive morphological 
characters are, in the opinion of a competent systematist, sufficiently 
definite to entitle it, or them, to a specific name. Groups of higher or: 
_ lower rank than species can be defined in a similar way. Thus, a sub- 
‘Species is a community, or a number of related communities, whose dis- 
 tinctive morphological characters are not, in the systematist’s opinion, 
sufficiently definite to merit a specific name, but are sufficient to demand 
