198 LIFE ON THE EARTH. 



common shell like Spirifera striata, we find two 

 difficulties one practical arising from the want of 

 an adequate public collection, to which reference 

 might always be made, for foreign as well as British 

 fossils, for varieties of race, and examples of the 

 young and the aged, and of stunted, deformed, or 

 gigantic growth. The best figures and the best de- 

 scriptions will never suffice for determining specific 

 distinctions in particular cases of very variable 

 groups. This practical difficulty, of becoming per- 

 fectly acquainted with the accepted nomenclature, 

 is aggravated by the cloud of theoretical obscurity 

 which has been for some time condensing round the 

 origin and character of species. Are the groups 

 which we call species united by characters which 

 vary only within definable limits? Are these limits 

 of variation for a given species constant for all the 

 area of its province, and for all the period of its 

 geological range? Palaeontologists generally answer 

 in the affirmative, and the type-specimens in our 

 Museums appear to confirm it. But if instead of 

 these selections to exemplify differences we make 

 large collections to illustrate agreements, the species 

 (so-called) seem to lose their distinctness, and a 

 second, or even a third or fourth group of forms may 

 be fairly united to the first, before arriving at a real 

 circumscription available for space and for time. 



