OTHER THEORIES OF SPECIES-FORMING. 2Ot 



without any indication of further reduction, indicate a pecu- 

 liar cessation in the forthright working of germinal selec- 

 tion. Why should not the weak determinants of these weak 

 organs go completely to ground in the struggle ? 



Actual experimentation on the influence of food-supply 

 in development does not bear out the assumption on which 

 the theory of germinal selection rests. Weismann himself 

 gave the larvae of flies, and I have given the larvae of silk- 

 worms through their whole life-time, an abnormally small 

 food supply (in the case of the silkworms this supply was. 

 from one-fourth to one-eighth the amount normally eaten 

 by full-fed larvae), with the only result that the mature 

 individuals were dwarfed ; that all their parts were reduced 

 in size, but the actual size proportions of the various organs 

 and parts, and their relations to each other, were unchanged. 

 The determinants seemed to share equally the hardships of 

 short rations rather than a few of the stronger getting the 

 better of the weaker. From the eggs of birds considerable 

 quantities of yolk have been withdrawn without modifying 

 appreciably the individuals developed from the eggs. 



If the struggle of the determinants is really an actual and 

 severe one then only those of the large strong organs should 

 survive, all the others being starved out. Such a condition 

 would result in the exclusive development of monsters, i. e.,. 

 individuals lacking numerous organs (the small ones), and 

 with the large ones all over-developed. 



Roux's Theory of Intra-sclection or the Battle of the- 

 Parts. Distinctly more likely to appeal to our reason is the 

 theory of Roux, 7 proposed in 1881, to explain how one or 

 more organs may exhibit a progressive development or 

 increase in size and capacity without reference to natural 

 selection and also to account for the many remarkable adap- 

 tations of slight and delicate but extremely precise character 

 exhibited by various internal organs. Roux made, however, 

 a too radical distinction between external or superficial 



