354 THE EVOLUTION THEORY 



It is, however, quite possible to leave aside for the present all 

 attempts at an explanation of life, and simply to take the elements 

 of life for granted, and on this basis to build up a theory of heredity. 

 We have already taken a step in that direction by establishing that 

 the whole substance of the fertilized ovum does not take part in 

 heredity in the same degree, but that only a small part, the chromatin 

 of the nucleus, is to be looked upon as the bearer of the hereditary 

 qualities, and by deducing, further, that this chromatin is made up of 

 a varying number of small but still visible units, the ids, each of 

 which virtually represents the whole organism, or, as I have already 

 expressed it, each of which contains within itself, as primary 

 constituents, all the parts of a perfect animal. 



It was these ' primary constituents ' which led us to the 

 digression in regard to Bonnet's theory of ' Evolutio ' and Wolffs 

 ' Epigenesis.' 



Let us now inquire what must be the constitution of such 

 a chromatin globule, an id, so that, shut up within the nucleus of 

 a living reproductive cell, it can direct the development of a new 

 organism which resembles its parent. Two fundamental assumptions 

 present themselves, and these can be related to every conception of 

 a ' germ-plasm,' even independently of the assumption of ids. Either 

 Ave may think of the id as made up of similar or of different kinds of 

 parts, none of which has any constant relation to the parts of the 

 perfect animal, or we think of it as composed of a mass of different 

 kinds of parts, each of which bears a relation to a particular part of 

 the 'perfect animal, and so to some extent represents its ' primary 

 constituents ' (Anlagen), although there may be no resemblance 

 between these ' primary constituents ' and the finished parts. The 

 assumption of a germ-plasm composed of similar parts, which has 

 been made, for instance, by Herbert Spencer, may be called the 

 modern form of epigenesis, while the other assumption is the modern 

 form of the ' evolution ' theory. As the former theory can no longer 

 call to its aid a ' formative power ' as a Deus ex machina, it can only 

 explain development as induced by the influence of external 

 conditions — temperature, air, water, gravity, position of parts — upon 

 the chemical components of the germ-plasm, which are everywhere 

 uniformly mingled ; and it makes no difference whether this uniform 

 germ-plasm is thought of as composed of many different kinds of 

 parts, as long as those parts are mingled uniformly to make the germ- 

 plasm and bear no relation to definite parts of the developing animal. 

 Oscar Hertwig has recently outlined such a theory. Although I cannot 

 expound it here I must say at least so much with regard to it, and to 



