EVOLUTION OF THE COLORS OF BIRDS. 17 



the reverse is impossible. From the destructive, it may 

 now be well to turn to the constructive form of criticism. 

 There have been two theories recently advanced in 

 opposition to Weismann's one by J. A. Ryder and the 

 other by Lloyd Morgan. Ryder has presented his 

 theory in an article entitled " A Physiological Hypothe- 

 sis of Heredity and Variations," in the "American 

 Naturalist.* He considers Weismann's views to be 

 fanciful and visionary and admits of no possible com- 

 promise. He agrees with Morgan that the introduction 

 of a hypothetical germ-plasm, instead of making the 

 question of heredity more simple, in reality complicates 

 matters. He believes with H. Milne Edwards and Hux- 

 ley that in the division of labor of the various cells of 

 the body, all have been specialized beyond the point 

 where further embryonic development is possible, with 

 the exception of the reproductive cells which remain un- 

 specialized, and hence capable of development. Spencer 

 confirms this view of the simplicity of the reproductive 

 cells. He says: f" The marvellous phenomena initiated 

 by the meeting of the sperm-cell and germ-cell, natur- 

 ally suggest the conception of some quite special and 

 peculiar properties possessed by these cells. It seems 

 obvious that this mysterious power which they display, 

 of originating a new and complex organism, dis- 

 tinguishes them in the broadest way from portions of 

 organic substance in general. Nevertheless, the more 

 we study the evidence the more is this assumption 

 shaken- the more are we led towards the conclusion 

 that these cells have not been made by some unusual 

 elaboration, fundamentally different from all other cells, 

 * * * the organs for preparing sperm-cells and germ- 



*Vol. 24, p. 85. 



t Principles of Biology, I, pp. 219-220. 



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