616 SCIENTIFIC THOUGHT. 



is derived from a corresponding piece of some older one, 

 as a lintel was derived from a lintel, a column from a 

 column, a piece of wall from a piece of wall. . . . We 

 appear to be severally built up out of a host of minute 

 particles of whose nature we know nothing, any one of 

 which may be derived from any one progenitor, but which 

 are usually transmitted in aggregates, considerable groups 

 being derived from the same progenitor. It would seem 

 that while the embryo is developing itself, the particles 

 more or less qualified for each new post wait, as it were, 

 in competition to obtain it. Also that the particle that 

 succeeds must owe its success partly to accident of posi- 

 tion and partly to being better qualified than any equally 

 well-placed competitor to gain a lodgment. Thus the 

 step-by-step development of the embryo cannot fail to be 

 influenced by an incalculable number of small and mostly 

 unknown circumstances." l 



Now, wherever we have to do with a very large 

 number of unknown elements which combine to produce 

 a result, we are introduced to those conditions with 

 which the theory of averages and probability deals. The 

 curve of error discovered by Laplace and Gauss to 

 picture the distribution of a large number of observations 

 around the average or mean position, which is taken as 

 the most probable or correct one, comes in as a valuable 

 aid, not in studying the errors of natural growth, but as 

 the graphical illustration of the deviations or variations 

 which cluster around what we call the normal, or with 

 Quetelet the mean, figure. Only the interest is now 

 attached not so much to specifying and defining the 

 1 ' Natural Inheritance,' p. 9. 



