EVIDENCES OF ORGANIC EVOLUTION 31 
do better than recapitulate, with only slight modifi- 
cations, the arguments there given : 
_I. THE GRADATION OF ORGANISMS.—Both in the 
animal and vegetable kingdoms we may trace, in spite 
of certain gaps, a long series of gradations in com- 
plexity of structure, so that between the simplest and 
the most complicated of living things a great number 
of intermediate stages are to be found. When we 
pass to the lower end of the scale in either case, we 
come upon a group of creatures of comparatively 
simple organization. Among them we find members 
with regard to which we cannot definitely say that 
they are either animals or plants. Moreover, these 
unicellular organisms resemble in many ways the 
egg-cell from which every individual among the higher 
animals and plants originates. 
It is true that we now know it to be quite impossible 
to dispose all the members of the animal kingdom in 
a single linear series, such as was formerly suggested, 
passing in orderly sequence from the amceba up to 
man. ‘ Instead of regarding living things as capable 
of arrangement in one series, like the steps of a ladder, 
the results of modern investigation compel us to dis- 
pose them as if they were the twigs and branches of a 
tree. The ends of the twigs represent individuals, 
the smallest groups of twigs species, larger groups 
genera, until we arrive at the source of all these 
ramifications of the main branch, which is represented 
by a common plan of structure.’ 
2. EMBRyYOLoGY.—AII the members of a particular 
group of animals or plants as a rule resemble one 
