778 A TEXTBOOK OF PHYSIOLOGY 



Heredity. It is a familiar proverb that " like produces like." 

 " Men do not gather grapes of thorns, or figs of thistles." To 

 account for this continuity of species, various hypotheses have from 

 time to time been propounded. It was at one time believed that 

 a miniature animal existed preformed either in the spermatozoon 

 or, more probably, in the ovum. Microscopy showed that such was 

 not the case. The view of " epigenesis " states that in the egg, 

 which is entirely different from the structure of the adult, there is 

 a successive formation of new parts which do not exist as such 

 within the egg. That like should produce like there must, however, be 

 some directing force within the egg. Darwin believed that the parents 

 contributed minute particles of all their own tissues to the reproductive 

 cells, and thus secured physical continuity of species the theory of 

 pangenesis. 



The most commonly accepted view is that in the simple repro- 

 ductive cells there exist, probably in the chromatin content of the 

 nucleus, complexes which determine the course of development of 

 the fertilized ovum. These germ cells themselves were produced 

 from the pre-existing germ cells of the fertilized ovum from which 

 each parent developed. The somatic cells of the developing embryo, 

 and therefore of the adult, are in reality the custodians of the germ 

 cells. They do not form new germ cells; they merely contribute to 

 their growth and development. The germ plasm is continuous from 

 one generation to another. 



Since nuclear material is contributed by both parents, and since 

 in the formation of the germinal elements such material undergoes 

 reduction in amount by a special method of cell division, it affords 

 an adequate explanation of why like should produce like, and yet 

 at the same time why there should be such a marked difference between 

 the offspring and the parents, and also between offspring themselves. 

 A litter of puppies resembles its parents, but there may be marked 

 variations in colour, temperament, and other characteristics of the 

 puppies. 



The question arises as to how these variations are to be accounted 

 for. Are they to be accounted for by heredity or by environment ? 

 It is asserted that heredity plays a large part on th? plea that in the 

 case of the new-born puppies the ante-natal environment has to all 

 intents and purposes been the same. But this is not so ; the condi- 

 tions, even in the womb, will no more be the same for each puppy 

 than they are for each egg in a mass of frog's spawn developing in a 

 pond. The slight variations in chemico -physical conditions may have 

 the profoundest effect on development. 



The further question arises as to whether environment after birth 

 can in any way influence hereditary characters. It is not a question of 

 gathering figs of thistles, but whether a bad fig-tree can by environment 

 be made to yield a strain of good figs. This is a question of great 

 importance to the sociologist. Lamarck asserted that " all is pre- 

 served in reproduction and transmitted to the offspring, that Nature 

 has made individuals to acquire or to lose by the influence of the 



