382 GENERAL SUMMARY TO 



into. When a form troubles the palaeontologist, it is usual in order to get over the difficulty 

 to make of it a " variety," whilst it is in reality no more than a modification in shape of 

 the same variable species. But, alas, the tendencies of palaeontologists have been to multiply 

 so-termed " species " and " varieties," and not to reduce them within reasonable limits. 

 Mr. John Young, who has always shown himself to be a careful and accurate observer, 

 writes to me that " When looking over Producius semireticulatus in his collection, Dr. L. 

 de Koninck expressed the opinion that some of the varieties or variations in shape ought 

 to be considered species ; but I find that some of these varieties are only types of 

 Productus semireticulatus that characterise this species in certain localities and on 

 different geological horizons, and that it is seldom we find more than one of the 

 varieties present in any of the particular beds in which the species is found. I 

 could almost tell, without knowing the locality, the geological horizon of many of the 

 varieties of our Scottish P. semireticulatus during the long life-history of the species 

 from the Lower Calciferous Sandstone up to the Upper Limestone of our coal-field ; and 

 it is to be expected that varying conditions of sea-bottom would have some influence on 

 the growth and form of the specimens, and that in the numerous returns of the species 

 over the same tracts of sea-bottom, but on higher and higher horizons of strata, new 

 varieties, or distinct races of the shell, if we may so term them, would naturally appear. 

 The numerous alternations of land-surfaces and old sea-bottoms during the deposition of our 

 Scottish Limestone strata would favour the introduction of these varieties more than in those 

 sea-bottoms where the conditions remained equal over longer geological periods of time." 1 

 Marie Rouault observes 2 . . . . " Nothing is more difficult to describe than a species 

 of which all the individuals representing it differ among themselves in as marked a 

 manner as this before us. Notwithstanding the difficulties that they cause, these differ- 

 ences may however be of very great use in the solution of the problem arising from them ; 

 for these differences, so strongly marked, observed between individuals of identical origin 

 must have a cause, considering that, here as everywhere, varieties always result, either 

 from different points of departure, or from the conditions in which they existed not 

 having been exactly similar ; if these differences continue they perpetuate the variety, 

 which would then become a new perfectly-defined species." Therefore in the author's 

 view all real varieties are the commencement of species, to perpetuate which it would 

 be sufficient to maintain the conditions which caused their existence ; and this also is 

 the view of the case so ably expounded and maintained by Darwin. Under such 

 conditions we should have real varieties, but they would differ materially from mere 



1 See a very instructive paper by Dr. L. de Koninck, " Sur la distribution geologique des fossiles 

 Carboniferes de la Belgique " (' Annales de la Soci^te ge'ologique de Belgique,' vol. ix, p. 50, 1883. In this 

 paper the learned Belgian palaeontologist shows that the Carboniferous Limestone of Belgium and of Great 

 Britain is divided into three distinct horizons, each of them being characterised by a distinct fauna. 



2 ' (Euvre8 posthumes de Marie Rouault,' par P. Lebesconte, 4to, Rennes and Paris, 1883; 'Notice 

 sur les Amorphozoaires du Terrain Silurien de la Bretagne,' p. 21. 



