92 INVERTEBRATE PALAEONTOLOGY 



Besides complication produced by geological imper- 

 fections, intrinsic difficulties obscure the trend of evolu- 

 tion. Like a river-system flowing over a broad plain, 

 the stream of organic progress is not confined within 

 one channel, nor does it follow a direct course. It is no 

 easier to identify the main current (if such can be said 

 to exist) in a delta, than to distinguish between the 

 divergencies and convergencies in evolutional history. 

 A general direction of progress can be assumed in the 

 one case no less than in the other ; but the bourne to 

 which evolution is tending has not yet been attained, 

 its source is somewhere in the terra incognita of the 

 remote past, while the complexities revealed by every 

 glimpse available demand close and detailed study before 

 their relation to the main system can be appreciated. 

 The guiding force that determines the flow of a river is 

 that of gravitation ; to that principle all observed features 

 can be referred, and by it unseen qualities can be con- 

 jectured. But what is the controlling agent that steers 

 the course of organic evolution ? That one exists is 

 beyond doubt ; its nature and operation are, as yet, but 

 dimly visible. 



Just as a group of organisms is the sum of its indi- 

 viduals, so an " individual " is the sum of its organs- 

 At any one time, a group may include precocious and 

 backward members ; every individual possesses struc- 

 tures in varying states of efficiency. The ' quality, or 

 stage of evolution, attained by a stock must be calcu- 

 lated as the average for its components, above and below 

 which the relatively scarce progressive and regressive 

 members flourish or languish. A group is in the 

 ascendant if the progressive members outnumber the 

 regressive ; it is decadent if the proportions are reversed, 

 for the average must tend to move towards the pre- 

 dominant extreme. The degree of success reached by 



