56 THE ORGANIC GROWTH OF THE LIVING WORLD sec. 



long periods in the direction of more complex (higher) 

 organisation, especially with the aiding action of selection. 

 But such higher development will be in a greater degree 

 experienced when an unusual increase of stimulation occurs 

 — in both cases as a consequence of modification, that is, 

 intensification of gi'owth. 



For the action of continuous unvarying stimuli the 

 influence of the use of an organ may serve as an example ; 

 further, the effect of cold and other climatic conditions which 

 produce hardiness. For the action of increased stimuli, the 

 influence of warmth on plants as it manifests itself in several 

 of their characters in various warm regions of the earth. 



Similarly, permanent cessation or diminution of the stimuli 

 will l^e followed by simplification of organisation as a result 

 of change of growth. 



Whenever the power of growth, be it through persistence 

 of characters, through retrogression, or through progress, has 

 undergone a change in any species under particular external 

 conditions, then the reapplication of a stimulus — even if it 

 be that which was formerly the most usual — will have a new 

 ground for action, will excite a peculiar kind of growth, and 

 lead to new structure. 



We have, however, to answer the question. Why male 

 animals for the most part are in advance of the females in 

 organisation, why, again, the older males first possess the new 

 characters, and why this new formation of characters (advance 

 in organisation) takes place on the body in a constant direc- 

 tion, for the most part from behind forwards ? And since the 

 changes in markings consist principally in the conversion of 

 a longitudinal striping into spots, and these into transverse 

 stripes, we have to inquire what are the causes of this con- 

 version. What are in general the causes of the modification 



