92 TEE POPULAR SCIENCE MONTHLY 



up of a flat layer of tissue on the yolk. Obviously in this case the gut 

 did not exist as such in the germ. 



It is unnecessary to multiply objections to this interesting bit of 

 metaphysic. Both the epigenetic and the preformationist theories of 

 the eighteenth century are dead and buried under the relentless logic of 

 events. Essential forces and preformed miniatures, alike in their 

 finality, were unable long to withhold the attention of naturalists from 

 the more potent suggestions of a rapidly growing body of new obser- 

 vations. 



With the discoveries that organisms are built up of morphologically 

 equivalent protoplasmic units, or cells; that both egg and sperm are 

 cells, also; that the nucleus, especially the chromatic substance, is the 

 part of the cell chiefly if not wholly concerned with the inheritance of 

 the individual and specific characters and their distribution in the 

 developing organism; more than all, with the discovery of the essential 

 nature of fertilization, new theories were devised to interpret the still 

 puzzling problem of individual and specific differentiation. These, like 

 their prototypes of the previous century, fall into two contrasting 

 classes. 



Both of these classes of theories recognize that individual differentia- 

 tion can not be interpreted without regard to race development. The 

 germ from which the individual springs has history behind it, is com- 

 posed, indeed, of two fragments of two preexisting individuals, the 

 parents, who, in turn, sprang similarly from a previous generation. 

 It is at once apparent that all modern theories of development must 

 reckon with these facts ; which means that, however simple we may con- 

 ceive a given germ to be, the probabilities are overwhelmingly opposed 

 to the conception that it is homogeneous ; and they are equally in favor 

 of the conception that it possesses from the start, in view of its relation 

 to a preexisting parent, some degree of differentiation. 



In perfect accord with these requirements, modern epigenesis and 

 modern preformation nevertheless exhibit characteristic differences. On 

 the one hand, is the preformationist theory of determinants devised 

 especially to explain the persistence, through many generations, of very 

 trifling characters, such, for instance, as a small pit on a human ear, 

 recognized as a family trait, or a spot on one surface of a butterfly's wing, 

 or a lock of white hair on a particular area of an otherwise dark-haired 

 head. Such characters appear to come and go without effecting in any 

 way the other characters of the organism. TJiis independent variability 

 is interpreted on the assumption of fundamental living units in the 

 chromatin of the germ nucleus that represent and determine all the 

 various characters of every individual. The germ chromatin is accord- 

 ingly conceived to contain the determinants of all the heritable charac- 

 ters; and these are further conceived to be so associated, that in the 



