76 CALIFORNIA ACADEMY OF SCIENCES. 



considered of importance. It is the recognition of tlie 

 importance of physical and chemical causes, as the 

 originators of new parts. The environment produces 

 changes of which natural elimination destroys the least 

 advantageous, leaving only the fittest to survive. There 

 are metabolic rhythms in life. When the anabolic ten- 

 dency is in the ascendent life is non-progressive, or 

 tliere may even be retardation; but when the katabolic 

 tendency is in the ascendent, life progresses along lines 

 determined: first, by the nature of the variations, and 

 second, by the operation of natural selection or elimi- 

 nation. 



It may be of interest to note the bearing of the law of 

 phylogenic extent and destiny. In the growing tree 

 whenever its progress is checked in any direction, if the 

 resistance be not too great the foliage becomes dense and 

 matted. So with life as a whole. When a new plan 

 of organization was originated, nature ran riot for a 

 time in the wealth and multiplicity of forms she dis- 

 played. Take for example the age of reptiles. In their 

 palmy days they were without competitors; climate and 

 vegetation conspired for their well being; the soil had 

 not been cultivated and was capable of bearing an im- 

 mense crop. This was a period of acceleration. There 

 was no resistance to the upward progress of growth-force 

 and it advanced rapidly along certain lines. Having 

 originated this great variety and multiplicity of forms 

 quite suddenly, comparatively speaking, a barrier was 

 presented to their future progress. The climate may 

 indeed have remained as propitious and the food as 

 abundant, yet an obstacle to their advance was intro- 

 duced. And this obstacle was apparently simply the 

 fact that they had reached the goal of their specializa- 

 tion. The growth-force was still there to be expended 

 and was used in increasing density or mass to speak 



