6 GENERAL EVOLUTION 



although the origin of most species by descent has not been ob- 

 served, every one knows the worthlessness of argument based on a 

 negative. Unless these cases exhibit opposing evidence of a posi- 

 tive character, they are absolutely silent witnesses. 



He who cites them against evolution commits the error of the 

 native of the Green Isle who testified at a murder trial. '^ Al- 

 though the i^rosecuting attorney brought three witnesses to swear 

 positively that they saw the murder committed, I could produce 

 thirty who swore they did not see it done ! " 



By the inductive process of reasoning we transfer the unknown 

 to the known, for it is the key of knowledge. It rests upon the 

 invariability of Nature's operations under identical circumstances, 

 and for its application merely demands that analysis and com- 

 parison shall fix that the nature of that of which something is 

 unknown is identical with that of which the same thing is 

 known. We then with certainty refer that which is known as 

 an attribute of that object of which the same quality had been 

 previously unknown. The following form exhibits its applica- 

 tion to the question of evolution. As preliminary facts it may 

 be assumed that : 



1. Many species are composed of identical elemental parts 

 which present minor differences. 



2. Some of these differences have been seen to originate spon- 

 taneously from parents which did not possess them, or, what is the 

 same thing, are known to exist in individuals whose parentage is 

 identical with others which do not possess them. 



3. The gradation of differences of the same elemental parts is 

 one of degree only, and not of kind. 



4. Induction. — Therefore all such differences have originated 

 by a modification in growth, or have made their appearance with- 

 out transmission in descent. 



II. THE MAN'NER OE EYOLUTIOK. 



In discussing this point, new evidence in favor of develop- 

 ment must be produced, and some statements of the history of 

 the opinion made. 



The laws which are expressed by all that we find of structure 

 in animals are four, viz. : 



1. Homology. — This means that animals are composed of cor- 

 responding parts ; that the variations of an original and fixed 

 number of elements constitute their only differences. A part 



