CHAPTER I. 
THE THEORY DEFINED AND TESTS PROPOSED. 
“ Old things need not be therefore true, 
O brother men ! nor yet the new. 
Ah ! still awhile the old thought retain, 
And yet consider it again.”—A. Clough. 
By what process have all the different kinds of animal 
and vegetable organisms become what we now see them 
to be ? This is the problem which has from time to time 
excited the interest of solitary thinkers, and which, within 
the last thirty or forty years, has profoundly agitated the 
scientific world. Two answers have been given to this 
question. On the one hand, it used to be asserted (it is 
still asserted by some) that the different kinds of animals 
and plants are the lineal descendants of organisms similar 
to themselves, and that those remote ancestors suddenly 
appeared upon the earth, some six thousand years ago, 
endowed with the power of reproducing their like from 
generation to generation. On the other hand, it is con 
tended that all existing kinds of organisms, however 
complex, have been produced by successive changes from 
the simplest and least complex forms of life. The latter is 
the opinion of the majority of scientific men at the present 
day. This process is designated by the term Organic 
Evolution : and evolution is defined by Mr. Herbert Spencer 
as the result of the joint action of growth or increase in 
size, and of development or increase in [the complexity 
of] structure.* 
The Principles of Biology, i., p. /jj, note.