ASPECTS OF BIOLOGY AND METHOD OF BIOLOGICAL STUDY. 35 
which the development must pursue. Without it the results gained 
woula be but a confused assemblage of isolated facts and discon- 
nected phenomena; but, aided by a philosophic method, the ob- 
served facts become scientific propositions ; what was apparently 
insignificant becomes full of meaning, and we get glimpses of the 
consummate laws which govern the whole. 
Classification an Expression of Affinities.— Hitherto we have 
been considering the individual organism without any direct refer- 
ence to others. But the requirements of the biological method 
can be satisfied only by a comparison of the various organisms one 
with the other. Now the grounds of such comparison may be va- 
rious, but what we are at present concerned with will be found in 
anatomical structure and in developmental changes; and in each 
of these directions facts of the highest order and of great signifi- 
cance become apparent. 
By a carefully regulated comparison of one organism with an- 
other, we discover the resemblances as well as the differences be- 
tween them. If these resemblances be strong, and occur in impor- 
tant points of structure or development, we assert that there is an 
affinity between the compared organisms, and we assume that the 
closeness of the aflinity varies directly with the nlopenoss of the 
resemblance. 
It is on the determination of these affinities that all philosophic 
classification of animals and plants must be. based. A philo- 
sophical classification of organized beings aims at being a succinct 
statement of the affinities between the objects so classified, these 
affinities being at the same time so set forth as to have their vari- 
ous degrees of closeness and remoteness indicated in the classifi- 
cation. 
Affinities have long been recognized as the grounds of a natural 
biological classification, but it is only quite lately that a new sig- 
nificance has been given to them by the assumption that they may 
indicate something more than simple agreement with a common 
plan— that they may be derived by inheritance from a common 
ancestral form, and that they therefore afford evidence of a true 
blood relationship between the organisms presenting them. 
The recognition of this relationship is the basis of what is 
known as the Descent Theory. No one doubts that the resem- 
blances we notice among the members of such small groups as 
those we name species are derived by inheritance from a common 
