356 SECTIONAL TRANSACTIONS.— C. 



17. Joint Discussion with Sections D and K on The Conception of a 

 Species. Opened by Dr. F. A. Bather, F.E.S. 



Other speakers : Prof. E. B. Poulton, F.R.S., Mr. A. J. VVilmott, 

 Prof. H. L. Hawkins, Dr. C. Tate Regan, F.R.S., Prof. J. 

 McLean Thompson, Dr. A. E. Trueman, Major C. C. Hurst. 



Dr. F. A. Bather. — Is the Species Concept of Value ? 



The answer depends on what the species concept is. A species concept will always 

 be necessary for practical convenience. The question is whether this practical species 

 concept corresponds either to any grouping of the natural facts or to that grouping 

 which we desire to emphasise. There is a concept towards which the facts of 

 paleontology seem to impel us, and another concept deducible from facts of genetic 

 experiment. Both may be true, as corresponding to certain realities, but it does 

 not follow that either should oust the existing systematic concept. 



Prof. Herbert L. Hawkins. — An analysis of the difficulties introduced by the 

 nature of fossil material, and the consequent inevitable differences between the 

 palaeontological and neontological view of a species. 



The contrasted attitude of the stratigrapher and the palseobiologist. 



Problems raised by chronological sequence and comphcated by palaeogeographical 

 characters. Continuity and convergence in evolution show the artificiality of 

 classification, and make difficult the selection of an arbitrary basis for it. The 

 introduction of structures of prime physiological importance seems fairly rapid, 

 and such episodes might furnish the framework of a systematic scheme. But from 

 the neontologists' standpoint such changes would mark generic or higher grades of 

 classification. 



In palaeontology the genua must be the unit ; the so-called species (when not of 

 generic rank or representing groups known very inadequately) are indefinite sections 

 of continuous lineages. 



Prof. J. McLean Thompson. — The present conception of a species is based 

 primarily on the characters of the adult organism. Such characters are more readily 

 observed than understood. An attempt will be made to show that by an onto- 

 genetic study new light is thrown on specific characters as to their time of 

 declaration during development and as to their interpretation. By this means a 

 re- valuation of specific characters may be obtained. 



Dr. A. E. Trueman. — The problems of species-nomenclature in palseontology are 

 perhaps most complicated in the case of those invertebrates which occur abundantly 

 at many horizons, and which exhibit wide variation at each horizon. This is 

 especially true if specimens at one horizon differ among themselves in the stages 

 attained in those characters in which the stock is progressing ; in such cases, although 

 the average degree of progress will be greater at a higher horizon, individual specimens 

 may be indistinguishable from certain of those at the lower horizon. 



Statistical investigation of extensive series of Carbonicola and Anthracomya (Coal 

 Measures) and Grrjphaa (Lias) shows that the variation at any horizon is continuous, 

 that each community is homogeneous and indivisible, and that the unit-characters, 

 on the whole, vary independently. Such a ' community ' may perhaps be compared 

 with an ' impure species ' or a group of more or less freely interbreeding individuals 

 with interwoven pedigrees. Nevertheless, the variations within the group are not 

 sharply separated as in Jordanons, but merge continuously into one another. 



It follows therefore that such an evolving stock must be regarded as a ' plexus ' 

 or a bundle of anastomosing lineages. A species can only be regarded as a fixed 

 point in such a plexus, and a specific name should only be applied to those specimens 

 which agree with the type in all determinable characters. Any other application 

 involves division into unnatural groups. 



Major C. C. Hurst. — Recent genetical experiments and cytological observations 

 have given a more precise definition of a species. A species is a group of individuals 

 of common descent with certain constant characters in common represented in the 

 nucleus of each cell by constant and characteristic sets of chromosomes. Two 

 thousand eight hundred and forty-five species of plants and animals, including repre- 

 sentatives of all the Phyla, so far examined, show remarkable constancy in their 

 specific sets of chromosomes. 



