726 THE POPULAR SCIENCE MONTHLY. 



birds as we now know them. They were, on the contrary, birds so rep- 

 tilian in character, that there is still some doubt whether bird-characters 

 or reptilian characters predominate in the mixture, and therefore 

 whether they ought to be called reptilian birds or bird-like reptiles. 

 From this common stem, the more specialized modern reptiles branched 

 off in one direction and typical birds in another, and intermediate 

 forms became extinct ; until, now, the two classes stand widely apart, 

 without apparent genetic connection. This subject will be more fully 

 treated hereafter, and other examples given. These two will be suffi- 

 cient now to make the idea clear. 



Such early forms combining the characters of two or more groups, 

 now widely separated, were called by Agassiz connecting types, com- 

 bining types, synthetic types, and sometimes prophetic types ; by Dana, 

 comprehensive types ; and by Huxley, generalized types. They are 

 most usually known now as generalized types, and their widely-sepa- 

 rated outcomes specialized types. Thus, in general, we may say 

 that the widely-separated groups of the present day, when traced 

 back in geological times, approach one another more and more until 

 they finally unite to form common stems, and these in their turn unite 

 to form a common trunk. From such a common trunk, by successive 

 branching and rebranching, each branch taking a different direction, 

 and all growing wider and wider apart (differentiation), have been 

 gradually generated all the diversified forms which we see at the 

 present day. The last leafy ramifications flower-bearing and fruit- 

 bearing of this tree of life, are the fauna and flora of the present 

 epoch. The law might be called the law of ramification, of speciali- 

 zation of the parts, and diversification of the whole. 



b. Many imagine that progress is the one law of evolution ; in fact, 

 that evolution and progress are coextensive and convertible terms. 

 They imagine that in evolution the movement must be upward and 

 onward in all parts ; that degeneration is the opposite of evolution. 

 This is far from the truth. There is, doubtless, in evolution, progress 

 to higher and higher planes ; but not along every line, nor in every 

 part : for this would be contrary to the law of differentiation. It is 

 only progress of the whole organic kingdom in its entirety. We can 

 best make this clear by an illustration. A growing tree branches and 

 again branches in all directions, some branches going upward, some 

 sidewise, and some downward anywhere, everywhere, for light and 

 air ; but the whole tree grows ever taller in its higher branches, larger 

 in the circumference of its outstretching arms, and more diversified in 

 structure. Even so the tree of life, by the law of differentiation, 

 branches and rebranches continually in all directions some branches 

 going upward to higher planes (progress), some pushing horizontally, 

 neither rising nor sinking, but only going farther from the generalized 

 origin (specialization) ; some going downward (degeneration), any- 

 where, everywhere, for an unoccupied place in the economy of Nature, 



