ie. . surroundings, supply an 
848 A. Hyatt—Biological Relations of the Jurassic Ammonites. 
so far as they distinguish the groups from each other. These 
may be often followed back to varieties of one species, showing 
that certain varieties have given rise to the groups. j 
varieties are few as compared with the whole number of varie- 
ties traceable in these original ancestral species. : 
Thus it seems clear, that these varieties must have haa cet- 
into a group or series of genetically connected forms. 
ke or the sur- 
ifferences to the law of natural selection, but limit its action 
strictly to the modification of the structural differences which 
tend to appear first in the varieties and then by inheritance 1p 
larger and larger groups and at earlier and earlier stages 10 at 
life of the individual. oe 
_It may also be shown by Cope’s law of the origination © 
differences by growth that the origin of these differences prob- 
ably lies in some law of growth under the influence peed sical 
dkind of food, climate, ete. us they 
