MARINE CRUSTACEANS. 449 



in the present instance do not prove, they only point to, the presence of Continuous Varia- 

 tion : but I do maintain (and the more I have examined the specimens, the more has my 

 conviction been strengthened) that such evidence as there is compels us, for the present at 

 any rate, to recognise that the Variation within the variety is Continuous. The actual proof 

 of this point must depend on the examination of a very large number of specimens ; and 

 it seems to me that, when this examination has been made, it will be found to confirm the 

 opinion suggested by the more slender evidence at present at our command. Assuming then 

 that we have here Continuous Variation, or considering any other case in which we know 

 we have similar variation, it seems desirable that the two extremes in such a case which, 

 if the variation were Discontinuous, would be designated as varieties, should also be denoted 

 by some analogous expression. For purposes of general discussion, of course, a precise word 

 is not absolutely necessary, some such expression as 'the two extremes.' expressing the point 

 at issue sufficiently clearly ; and it is equally possible to distinguish the two extremes as 

 ' high ' and ' low ' respectively (cf. Bateson's Materials, pp. 39 — 40). But the practical con- 

 venience of the systematist often requires a definite symbol by which to designate a form 

 he is describing; it would not be enough, for instance, that he should designate a specimen 

 van glabrous without specifying whether it is one or other extreme of that variety, or one 

 of the intermediates. Of course it is not possible to sum up any given intermediate in a 

 word, but the extremes give us fixed points on which we may rely. I propose, therefore, to 

 use the word ' term ' to express the extremes of structural tj'jie manifested in a Continuous 

 Variation, and for purposes of systematy, it is only necessary to qualify the word by the 

 addition of, say, an alphabetical letter to distinguish the one term from the other. For the 

 word itself I would point out that it is brief, it is Latin in origin, that in its primary 

 meaning it expresses what it is wished to express, namely the existence of " ends " to a 

 series, and lastly but not leastly it is quite neutral, suggesting no more than it is meant 

 to suggest. The expression ' end-form ' is excluded by reason of the special meaning recently 

 attached to the word ' form ' ; the word ' extreme ' on the other hand is of so much use for 

 general purposes that it would only make confusion to give it a special meaning (as indeed 

 has happened now in the case of the word ' form '). ' End-term ' suggested itself, but is, 

 after all, tautologous. The words ' high ' and ' low ' have been already used by Mr Bateson 

 in a special sense, applying to the ends of a Discontinuous series in the case of the males 

 only of a species ; moreover they imply degree of variation. 



Assuming, then, the use of this word, two points call for special comment. Firstly, it is 

 plain that the use of the word can be of no help in the cases where it is an intermediate 

 that is being refeiTed to, and as by the nature of the case, the intermediates are more numerous 

 than the terms, it may be objected that the word will have but a very limited value. This 

 is true, but the value, limited though it be, seems to me a real value. The fact that we cannot 

 express by a word the differences between the intermediates does not alter the fact that we 

 can so express the often great structural differences between the two extremes. Were two 

 genera to be found to be united by a continuous series of intermediates the series would form 

 a single genus and species, but it would none the less be convenient to designate the ex- 

 tremes, with their great structural differences, by some special word, even though we could 

 not do the same in the case of any given intermediate which also differed greatly from one 

 of the extremes. 



Secondly, it is always possible that, though in a given series two extremes may be dis- 

 tinguished as ' term a ' and ' term 6,' later research . may show one or the other not to be 



