238 DISTRIBUTION OF THE MOLLUSCA IN TIME. 



Extension of Species^ Genera and Families. — " The distribution 

 of species in the fossiliferous beds is comparable to that of living 

 species in space. In fact, we find a bed where each species has 

 its maximum of numbers ; above and below this bed, it is only- 

 represented by less numerous individuals, or has disappeared. 

 If the locality where the first individuals of the species appeared 

 is very distant from that in which the last of them occur, one 

 can thus appreciate their geographical migrations during the 

 interval. 



" The study of the distribution of fossil genera, compared to 

 that of the same genera living, will give interesting results and 

 reveal considerable modifications in the condition of ancient seas 

 and continents. 



" We are led to believe that each genus has had a centre of 

 creation or of diffusion ;* or that (according to the development 

 theory), each genus has become constituted within a determined 

 region. The enormous geographical extension of several fossil 

 genera, has supported the supposition that there existed many 

 centres of creation, but the diffusion of embryonic marine 

 Malacozoa is so easy, that it appears useless to have recourse to 

 this hypothesis. 



" All the genera of mollusks are not equally plastic, nor 

 modified by time. Nautilus, Natica, Area, Nucula, Chiton, 

 Lingula, Terebratula, Rhynchonella, etc., have had a much 

 greater longevity than the others, and more or less resemble 

 living forms. Terrestrial and fluviatile mollusks are relatively 

 less changed than marine mollusks ; Melanopsis, Planorbis, Pupa 

 of the ancient beds, scarcely differ at all from living forms. 



" This resistance of certain mollusks to modification, contrasts 

 with the extreme plasticity of types of echinoderms, reptiles 

 and mammals, by which the thinnest stratigraphical horizons 

 can be characterized. 



" The cause of the persistence of these types is unknown. 

 To say that with them the law of heredity is stronger than the 

 law of variability, is not an explanation. Then again, many 

 genera of mollusks, after enduring through several geological 

 periods, suddenly become extinct. Others have made but an 

 appearance, so to say, upon the surface of the globe. These 



* L. Agassiz and Prof. E. Forbes have represented diagrammatically, 

 the distribution of genera in time, as well as their duration, by means of 

 a horizontal line crossing perpendicular columns representing the forma- 

 tions ; its left extremity touches where the genus first occurs, its right 

 extremity where it became extinct ; the line is swelled or thickened 

 according to the amount of development of the species in the various 

 strata. For example, a line thus -^^ indicates that the genus has 

 become extinct, and that it became most largely developed at the middle 

 period of its duration ; a line -^ indicates that the genus has been 

 devteloping to the present moment, ■which is, so far, its maximum. 



