i 9 4 SCIENCE PROGRESS 



Heredity 



On the earth are some half-million different species of 

 animals and plants, each of which breeds true in virtue of 

 what we call heredity. Each species therefore represents a 

 strain or line of descent of living matter always growing, 

 dividing, and increasing in mass, like the little Paramcecia we 

 have already considered, each striving to occupy the whole earth, 

 and restrained in the attempt only by the accident of death. 



The strains of living matter are separated from one another 

 by a wide gulf which we do not know how to bridge. Change 

 of state seems to be without effect. Continuous supplies of 

 the richest food will not convert a strain of dwarfs into giants. 

 In the solemn words of the Burial Service, " All flesh is not 

 the same flesh, but there is one kind of flesh of men, another 

 flesh of beasts, another of fishes, and another of birds." 



The nature of these differences in the kinds of living matter 

 and their mastery so that we may be able to control them is 

 without doubt the most difficult and the most important 

 problem which science has attempted to solve : most difficult 

 because it deals with a form of matter much more complex than 

 any which the chemist or physicist so far has considered ; 

 more important because on the solution of this problem 

 depends the possibility of removing practical medicine, politics, 

 and morality from the domain of empiricism and tradition to 

 that of rational co-ordinate knowledge. 



To speak of a strain breeding true is a bald way of 

 describing a force so potent as heredity, so impish in its 

 eccentricities. On the Antarctic ice there abound a race of 

 birds called penguins. They have never seen a tree since they 

 first were penguins ; they do not fly, for their wings have been 

 reduced to small flat paddles with which they swim. The 

 bird cannot tuck its head under its wing, because the wings 

 are too shrunken ; but still, in mute worship and touching 

 fidelity to its forbears of thousands of years ago, when it 

 composes itself to sleep each individual bends round its head 

 and tucks the tip of the beak — it is all it can do, poor thing ! — 

 under the dwarfed wing. 



This lingering instinct, this obsession by the great past, 

 is like a whale dreaming of the green fields in which his 

 forefathers browsed ! Now, each individual penguin starts life 



