128 BIOLOGICAL LECTURES. 



more profound and difficult, and fortunately not pertinent to 

 the immediate object of this lecture. 



This definition of evolution is based upon the whole mass of 

 phenomena which has been shifting and modifying itself before 

 the eyes of observers from the earliest times to the present day. 

 The fact of perpetual change is acknowledged, and it has also 

 been generally recognized that different bodies change in vari- 

 able degrees. Some substances are so elemental and primitive 

 in comparison with those succeeding them in time or in their 

 own series that they can be spoken of as relatively unchange- 

 able in comparison with these. The inorganic body that we 

 now speak of as the ether and some of the chemical elements 

 may be rationally imaged as still existent in unchanged condi- 

 tion in some parts of the universe, and in those particular cases 

 there would be continuity without change, and consequently no 

 evolution in the sense that this is generally used. 



It has also been quite commonly and reasonably supposed 

 that the most primitive of the existing forms of the animal 

 kingdom among Protozoa, Protamoeba, for example, or even 

 so comparatively complex an animal as the existing Amoeba, 

 may still retain most of the characters that they possessed when 

 near the beginning point in the evolution of organisms. 



It is difficult, in view of such possibilities, to avoid the theo- 

 retical conclusion that the evolution of organisms was quite 

 distinct in its first period, and the earliest primitive organisms 

 necessarily more like inorganisms and comparatively unchang- 

 ing in their structures. In other words, evolution probably 

 began to work gradually, and there was necessarily a time 

 in which the laws of organic evolution, as stated above, were 

 comparatively inoperative. Biology, like mathematics, has there- 

 fore an imaginary zero point in time and space, so far as vital 

 characters are concerned, and an actual unit of departure only 

 in the simplest organisms. 



Terrestrial organic bodies necessarily move forwards in time 

 and space with the earth, but they have also definite motions 

 of their own. There are two classes of these: (i) the move- 

 ments of the single organism through its different stages of 

 growth and development. These ontogenic changes are divisi- 



