14 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
specimens belong to the same species because the morphological char- 
acters which the zoologist regards as of specific rank are alike in the 
two specimens. The members of the . same order are thus classified 
together because they exhibit the same ordinal characters. The 
members of the same species were formally supposed to be so asso- 
ciated because of their genetic affinity — i. e., descent from common 
parents; but we are now accustomed to recognize community of char- 
acters, of whatever rank, as an indication of the genetic affinity of the 
organisms exhibiting them. The difference between ordinal affinity 
and specific affinity is one of degree, not of kind ; the members of the 
same order are genetically related, but the relationship is more distant 
than that of members of the same species. Thus the terms species, 
genus, order, and class are applied to aggregates of plants and animals 
on the basis of their genetic affinity, and the several terms indicate 
the degree of nearness of affinit}^. The individuals associated to form 
a particular aggregate of this kind may be fossils or living beings, 
and they may come from opposite sides of the earth, but they are 
associated on the basis of the likeness of the morphological characters 
they possess, and they are classified on the basis of the theoretical 
relative degrees of kinship they bear to one another. A species or a 
genus is therefore an ideal aggregate. No one ever sees the whole of 
a species, and only as its relationship to place and time are indicated 
can the aggregate called a species be defined. Furthermore, the terms 
species, genus, etc., are arbitrarily applied in every particular case. 
In other words, there is no standard except common practice to deter- 
mine what characters are of varietal, specific, or generic rank. But 
the law is well established that the aggregate shall be named in the 
order of degree of affinity by the terms species, genus, family, order, 
class, etc. , terms implying, progressively, near to more distant kinship. 
A second mode of classifying organic aggregates is on the basis of 
their relationship to environment, or to the conditions of life. Thus 
we find Walther, in his "Bionomie desMeeres" (1873), adopting and 
applying Haeckel's terms: Halobios, the total aggregate of living 
beings inhabiting the sea, as distinguished from Limnobios, the 
inhabitants of fresh water, and from Geobios, the organisms inhab^ 
iting the land. The marine organisms (Halobios) are subdivided 
into Benthos, those living on the bottom, as distinguished from Necton 
and Plankton, the inhabitants of the open seas. Depth of range of 
faunas or floras is indicated by such terms as littoral or abyssal. Such 
aggregates are made without consideration of genetic affinity or like- 
ness of form; all kinds of animals and plants living together are 
included. The general basis of the classification is coincident with 
area of geographical distribution, and the relationship determining 
the classification is the adaptation of the organisms to the common 
conditions of environment. 
A third kind of aggregates of organisms is defined by the geologist 
