A PROBLEM IN EVOLUTION 435 



upper and lower lips, may be regarded as the last outward expression 

 of this primitive condition (Fig. 9, C). Thus it appears that the 

 growth of the human face is still dominated by the same forces that 

 have been observed in the remote arachnid ancestors of the verte- 

 brate stock. 



There are many other organs of man, especially those in the head, 

 as, for example, the hypophysis cerebri, thymus, thyroid and pineal eye, 

 that have their origin in the arachnids, and it is there that we must 

 look for a better understanding of their significance. 



VII. Conclusion 



"When, in the manner indicated above, we have united the upper 

 and lower sections of the animal kingdom to form one great phylum or 

 highway of evolution, we observe an amazing uniformity and sta- 

 bility in the basic structure and mode of growth of all the animals 

 belonging to it. The direct course of evolution within such a definitely 

 and narrowly circumscribed path, during untold millions of years, and 

 under widely different conditions, clearly indicates, in my judgment, 

 that neither the haphazard shuffling of "hereditary units" in sexual 

 reproduction, nor external environment, nor use and disuse, nor natural 

 selection, has been the chief directing agent in organic evolution. 

 At best, they are of secondary, or subordinate, or intermittent value. 

 The primary and ever present creators of organic structure appear to 

 have been a universal, persistent power of growth, rigidly controlled 

 by the inherent nature of organizable materials ; and the internal con- 

 ditions successively created by growth and organic association. 



