444 Life and Immortality. 
of high classificatory value, and only embryological charac- 
ters are frequently the most valuable of all. The real affini- 
ties of all organisms, in contradistinction to their adaptive 
likenesses, are due to inheritance or community of descent. 
Hence, a natural system of classification is a genealogical 
arrangement, with the acquired grades of difference, denoted 
by varieties, species, genera, families, etc., and their lines of 
descent have to be discovered by the most permanent charac- 
ters, whatever they may be and how little of vital importance 
they may possess. 
That species are immutable productions, which was 
until quite recently the current belief by laymen and nat- 
uralists, was almost unavoidable so long as the world was 
considered to be of short duration. But now that some idea 
has been acquired of the time that has elapsed since the 
beginning of earth-life, we are too apt to assume, without 
proof, that the geologic record is so complete, that it would 
have afforded us some plain evidence of the mutation of 
species, if they had undergone mutation. But the principal 
cause of our unwillingness to admit that one species has 
given birth to other and distinct species, is that we are 
always slow in admitting any great change of which we do 
not discern the intermediate steps. Just such a difficulty 
was felt by many geologists when Lyell first insisted that 
long lines of inland cliffs had been produced, and great val- 
leys excavated, by the agencies which are still at work in the 
earth. No effort of mind can adequately grasp the meaning 
of even ten million of years, nor add up and perceive the full 
effects of the many slight variations to which species have 
been subjected during an almost infinite number of genera- 
tions. The day, however, is not distant, when mankind will 
have become just as thoroughly convinced that species have 
been modified during a long course of descent, mainly through 
the Natural Selection of innumerous successive, slight and 
favorable variations as they are that the attraction of gravi- 
tation is an important element in the maintenance of the 
