SECTIONAL TRANSACTIONS.—D. 399 
Tn all cases, from species to phyla, material originally indifferent or heterogeneous has 
been integrated into classifiable groups by the environment. It is, no less than 
descent, worthy of consideration as a basis of rational classification (see Presidential 
Address to 8.W. Nats. Union in its Proceedings, II, 73-91, January 1931). 
Dr. W. D. Lane, F.R.S. 
Increasing specialisation in Systematic Zoology leads to increasing difficulty in 
expressing classificatory facts in terms of Linnean nomenclature ; but, so long as 
Recent organisms only are dealt with, the difficulty is mainly one of complication, and 
no new principle is involved ; with certain reservations, morphic similarity is usually 
considered a safe criterion of phyletic affinity. On the other hand, when organisms 
are studied in the light of stratigraphy, and the Recent fauna is seen to be but a cross- 
section of innumerable lineages, then systematic units based upon Recent organisms 
often are seen to be polyphyletic, and the difficulties of fitting the supposed facts of 
phylogeny into an appropriate frame of orthodox systematic nomenclature become so 
formidable that one is tempted to divorce systematics from phylogeny. Rather 
than this it would be better to re-cast our system of nomenclature ; but this is not 
inevitable. The stage we are considering is this: A—-B—C—D are terms in a lineage. 
The lineage is now a genus, and the terms, species. With complete knowledge our 
species would tend to become arbitrary points in a continuous series. 
But a complication arises. We find parallel, often contemporary, lineages with 
corresponding terms ; and immediately there is the difficulty of diagnosing the genera. 
But still we should keep to principle, and act on the faith that it is our lack of percep- 
tion that prevents us from indicating a common character running through 
A—B—C—D by which the lineage differs from a—b—c—d ; even if in practice we 
can only distinguish the corresponding early or late terms, and have to define the 
lineage, that is the genus, as a unit composed of a term, or terms, possessing certain 
characters, and including other terms leading up to, or away from, that point. 
But a second complication arises. We are told that, in certain cases, parallel 
lineages interbreed ; and so, instead of a bundle of parallel lineages, we have an 
anastomosing complex. Here systematic nomenclature is up against a formidable 
difficulty. Still we must not sacrifice what we hold to be phyletic facts to the 
exigencies of systematic nomenclature. It is generally conceded that at any rate 
certain species of plants interbreed, or have lately interbred, and the hybrid races 
have become, or are becoming, new species. The fact is recognised, and the plant is 
given a new specific name. But in the case we are considering lineages (which we 
are calling genera) are involved. I cannot see any way but to consider the hybrid 
species as constituting new genera, and to name them accordingly. 
Dr. J. StepHEenson, F.R.S. 
While we must, of course, see to it that near relations are not too widely separated 
in our schemes of classification, experience shows that a classification may be useful, 
‘and yet be independent of phylogeny; moreover, we often know too little about 
phylogeny to use it for classification ; and where we do know the phylogeny, it would 
sometimes be highly inconvenient to express it in classification. As to convergence :— 
the particular group of morphological characters that distinguishes a genus has some- 
times been evolved in more than one way; in other words, a genus may have more 
than one ancestry (may be polyphyletic)—which again cannot be indicated in 
classification. 
Dr. Hue Scort. 
These remarks deal with the bearing of the subject under discussion on 
zoogeography. Zoogeographers are wont to base far-reaching conclusions on the 
occurrence of organisms of the same group in widely-separated parts of the world. 
A consideration of the geographical distribution of terrestrial animals often involves 
the supposition that either certain species or groups possess means of dispersal over 
the sea, or land connexions formerly existed between countries now separated by 
the ocean. Such conclusions are, however, plainly untenable if the members of a 
