II GROWTH IN DEFINITE DIRECTIONS 59 



marking live longest— in other words, bear the new markin^r 

 for the longest time, this becomes, as it were, most firmly 

 inoculated in the organism, and is therefore with greater 

 certainty transmitted to the offspring. The longer it is borne 

 by the possessors the more completely from constitutional 

 causes will it be inherited, and as the duration of its existence 

 in the posssessorwill be proportional to its usefulness, the degree 

 of its heredity will also be proportional to its usefulness, so that 

 an important share must be ascribed in the process to the con- 

 servative adaptation which depends on constitutional causes." 



But all this does not explain the first occurrence of the 

 new characters, nor the undeviating course of the evolution 

 in a particular direction. 



Tor when a number of varying individuals are compared 

 it is seen that the variations of all tend to a definite end, and 

 that the majority of the intermediate forms show stages in 

 the development of the characters which are absolutely with- 

 out use to them. 



Tliis cannot be explained except by natural growth, whose 

 operations are changed, intensified, or diminished to a certain 

 extent by the stress of adaptation, and may also at times be 

 entirely restrained. 



That this growth proceeds in definite directions and begins 

 at definite spots is not more wonderful than the same processes 

 in individual growth, which are of course more evident in 

 plants than in animals ; in the latter, on account of the more 

 active life, they are more altered and obscured by adaptation 

 than in the former. 



Some important ultimate causes of the direction of growth 

 must, however, be sought in the general physiological mechan- 

 isms of the body — for example, in the distribution of the blood, 

 etc. In particular, the tendency to symmetrical, and even to 

 metameric development in the marking, for instance, depends 

 obviously on such mechanisms. 



