98 Mutations and Evolution. 



This is obviously a field where more definite conceptions will only 

 be possible after much further experimental work. 



To mention a single case of experimental morphogenesis in 

 plants, Harper (1918) has carefully studied the organization and 

 reproduction of Pediastrum colonies, and concludes that from the 

 point of view of inheritance the characters are of different kinds. 

 The four-lobed shape of the cells of a colony he thinks may be an 

 adaptive character which arose as an environmental response to the 

 pressure relations between cells and has now become fixed and 

 transmitted by cell division. 



In the ostrich * there are two callosities on the ankle, one 

 median which appears before hatching, hence inherited ; the other 

 on the side, which appears only after the bird uses this surface to 

 rest upon and is not transmissible. It is indicated that the median 

 callosity is much older, dating back to the Pliocene ostrich, which, 

 having three toes, rested on its legs symmetrically and so developed 

 the median callosities which have since become inherited 

 independently of any external stimulus. This callosity is no longer 

 used, the loss of the third toe, according to Duerden, having led to 

 a shifting of the point of contact with the ground and the 

 development of a new callosity which is not transmitted. The 

 difference in inheritance of these two callosities is very difficult to 

 explain satisfactorily on any basis except that of functional 

 inheritance. 



Brief reference must also be made to two important papers by 

 Guyer and Smith* recently received. A fowl serum was prepared 

 sensitized to (i.e., containing a cytolysin which dissolved) the lens 

 of the eye of the rabbit. This serum injected into pregnant rabbits 

 produced inherited defects in the eyes of many of the young. 

 Experiments with mice gave similar results. In rabbits the defects 

 were transmitted for six generations, through the male as well as 

 the female, and were gradually intensified without any further 

 injections. The defect behaved in general as a Mendelian recessive. 

 Here is clearly a specific modification produced by extrinsic factors. 



In concluding this chapter, it is evident that conceptions of 

 functional inheritance in various forms are again making themselves 

 felt in much of the constructive thinking of the present time. 

 1 Duerden, J. E. Amer. Nat. Vol. 54, 1920. 

 * Journ. Exptl. Zool. Vols, 26, 31. 



