30 EVOLUTION 



1. THE GRADATION OF ORGANISMS. Both in the 

 animal and vegetable kingdoms we may trace, in spite 

 of certain gaps, a long series of gradations in com- 

 plexity of structure, so that between the simplest and 

 the most complicated of living things a great number 

 of intermediate stages are to be found- When we 

 pass to the lower end of the scale in either case, we 

 come upon a group of creatures of comparatively 

 simple organization. Among them we find members 

 with regard to which we cannot definitely say that 

 they are either animals or plants. Moreover, these 

 unicellular organisms resemble in many ways the 

 egg-cell from which every individual among the higher 

 animals and plants originates. 



2. EMBRYOLOGY. All the members of a particular 

 group of animals or plants as a rule resemble one 

 another more closely in the early stages of their indi- 

 vidual development than they do in the adult condi- 

 tion, and in the earliest stages of all they are often 

 indistinguishable. These facts are explained if we 

 suppose that such individuals have a common origin, 

 that they are descended from a common ancestor, and 

 that traces of their pedigree are still to be observed in 

 the developmental stages through which each one 

 passes. We do not find a complete parallelism be- 

 tween the development of the individual and the 

 history of the race, nor should we expect to do so ? 

 since embryonic as well as adult stages may be modified 

 in the course of evolution ; what we should expect is 

 a more or less vague historical sketch, and this is what 

 is usually found remaining. 



