dall] notes on some upper cretaceous VOLUTID.E 3 



The forms developed somewhat later in all the upper Cretaceous 

 areas which have been explored have a notable family likeness, 

 together with features which in each case lend a certain local facies 

 to the species of each special local fauna. We find also that among 

 the species which make up the group in each fauna are usually re- 

 peated certain types of form, each of which probably corresponds 

 to some special conditions which make it fittest to survive, while 

 each faunal locality probably includes about the same groups of con- 

 ditions each of which impinges upon a particular species or group of 

 species more effectively than on the others. To illustrate the case 

 metaphorically, it seems as if each faunal district resembles a temple 

 containing a number of niches of different shapes, in which the 

 species of each genus or family resident in the district are obliged 

 by the pressure of the environment and the action of natural selec- 

 tion to take their places, those which fail to conform to some one of 

 these protective and formative niches being unable to survive. 



Whether the types preserved by these conditions, with their pro- 

 nounced analogies of form and ornament, should be classed by 

 dynamically developed characters, when it is probable that their 

 genetic connections are closer with the local group rather than with 

 their analogues in other districts and exotic groups, is a subject 

 which naturally opens up the whole question of the proper relations 

 between classification and nearness of genetic ties. Those system- 

 atists who claim that degrees of genetic relationship should govern 

 classification, to the exclusion of all other factors, will have no diffi- 

 culty in deciding the question. Others, with perhaps greater appre- 

 ciation of the complexity of organic relations and who believe that 

 classification is a means by which we may obtain an end and not an 

 end in itself, must hesitate longer. Without losing sight of genetic 

 connections in a broad sense, in the present state of science at least, 

 it is more convenient, and not less suggestive to the student, to recog- 

 nize in the system the community of response to the environment 

 at a particular stage of evolution, as well as the more hypothetical 

 connections believed or suspected to conform to the "line" of de- 

 scent. It may even be doubted whether response to the environment 

 is not in many cases the more potent factor in evolutionary progress 

 than, the tendencies inherited from an ancestral reticulum ; for it is 

 certain that no organism is of purely, or even potentially, linear 

 descent for any long series of generations. 



The possibility of migration complicates the question somewhat, 

 though in geological horizons believed to be nearly contemporaneous 

 and representing equivalent stages of evolution it is probable that 

 migrations play a very minor part. 



