Earlier, we invoked two
factors, and their uncertain relations, in order to explain the diversity
within a stratum -- degrees of development or perfection and types of forms.
They now undergo a profound transformation. There is a double tendency for
types of forms to be understood increasingly in terms of populations,
packs and colonies, collectivities or multiplicities; and degrees of development
in terms of speeds, rates, coeffieients, and differential
relations. A double deepening. This, Darwinism's fundamental contribution,
implies a new coupling of individuals and milieus
on the stratum. [TP: 47-8]
Darwinism's two fundamental contributions move in the direction of a science of multiplicities: the substitution of populations for types, and the substitution of rates or differential relations for degrees. [TP: 48]
A stratum obviously presents very diverse forms and substances, a variety of codes and milieus. It thus possesses both different formal Types of organization and different substantial Modes of development, which divide it into parastrata and epistrata [TP: 502]