AND ZOOGEOGRAPHY OF 1NDJAX OLIGOCHJETA. 123 



As to the further point, that Megascolex is not a true genus 

 but a group of genera., the matter is largely one of words. If 

 anyone wishes to paraphrase my conclusion, and say that 

 "different lines of descent may give rise to forms that it is 

 impossible to separate generically by anatomical characters," I 

 have no quarrel with him. I believe, however, that the sentence 

 " genera may be polyphyletic " expresses this conclusion with 

 ecpial correctness. For what is a genus ? It is, of course, an 

 assemblage of species having certain characters in common : are 

 we justified in going further, and saying " A genus is an 

 assemblage of species having certain characters in common, and 

 owning a common descent " ? 



I do not think so. In the first place, the term genus was used 

 in the first sense long before the doctrine of descent had won 

 acceptance. Another and more important reason is that, as must 

 be evident from what has preceded, we do not know what the 

 lines of descent certainly are, even in so well known a group 

 (and one so favourable for our purpose) as the Megascolecidae. If 

 we choose the second conception, we shall find it impossible, in 

 the present state of knowledge, to divide up the polyphyletic 

 group known as the genus Megascolex into assemblages of species 

 having certain characters in common and owning a common 

 descent. 



Yet genera., definite assemblages, we must have ; we must have 

 groups above species, and these groups must necessarily have 

 limits of some kind. Since we cannot, in the present state of 

 knowledge, define these groups by their descent, we must define 

 them by their anatomical characters, and perhaps by their 

 distribution. In some cases we can say that in all probability a 

 group so defined is a genetic unity ; in many cases we do not 

 know Avhether this is so or not ; in some we shall suspect or feel 

 convinced that it is not. But till we are able definitely to mark 

 out new groups on genetic lines, we cannot relinquish the old 

 anatomical groups. 



A classification is one thing, a phylogenetic tree another. No 

 one believes more firmly than I do that phylogeny ought to be 

 the basis of classification ; but candour must admit that as yet 

 it is incapable, in many cases, of constituting such a basis. Our 

 classification must necessarily, for practical reasons, present itself 

 as a complete scheme ; our phylogenetic trees are and will long 

 remain woefully incomplete. As our ideas of phylogeny become 

 more and more settled, our classification must be x-e vised to 

 coxTespond with it. But so long a,s we see anatomical groups 

 which we suspect, or can demoxistrate, to be of diverse origin, 

 withoxxt being able definitely to separate them up according to 

 their descent, so long we shall have to pixt up with polyphyletic 

 gexiex-a. 



For bibliography see end of next section. 



