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INTRODUCTION. 13 
some died out and others survived ; whether these sur- 
vivors were modified, and, if so, by what means; why 
certain plants and animals are found in places not best 
suited to their structure; why similar forms of life are not 
always found under similar physical conditions, and why 
dissimilar plants and animals are often found under similar 
physical conditions; why animals have organs of no use 
to them; why some animals are protected, why others 
are not, etc. etc. We consider that these questions are 
answered by the theory of the Evolution of Life, and that 
this theory may be regarded as the fundamental truth of 
Biology. We propose in this essay to bring together, in as 
popular a manner as possible, some of the evidence in favor 
of this theory, endeavoring to show that the different 
plants and animals are linked together by transitional 
forms; that there has been a progress from the simple, 
lowly organized, to the highly complex forms of life; that 
the transitional stages through which a plant or an animal 
passes in the development from its primitive to its adult 
condition are permanently retained in the lower forms of 
life; that the development of the higher forms of life from 
the lower has been brought about by Natural Selection ; 
and that Man has descended from a lower extinct form, of 
which the Gorilla and the Chimpanzee are the nearest 
living representatives. 
Before discussing these different subjects, as there seems 
to be still some misunderstanding in reference to the views 
of the predecessors of Mr. Darwin, the most distinguished 
of the advocates of the Evolution of Life, it may be perhaps 
not superfluous to glance at the literature of the subject. 
Here and there among the writings of the ancients, one 
meets with passages, and even works, like those of Lucre- 
tius, from which it is evident that at different times some 
doctrine like that of the Evolution of Life was held by the 
thinkers of antiquity. Passing by the speculations of 
