Page:Popular Science Monthly Volume 82.djvu/439

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435
A PROBLEM IN EVOLUTION

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 vertebrate 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 stability 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 conditions successively created by growth and organic association.