Mutations and Evolution . 
239 
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 1 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 3 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. 
a Journ. Exptl. Zool. Vols. 26, 31. 
