370 THE STUDY OF ANIMAL LIFE CHAP. 



of a sponge, or of a polyp, or of a Planarian worm, and 

 so on, will readily develop into an entire organism, we 

 must recognise that in simple creatures at any rate the 

 contrast between body-cells and germ-cells is not hard 

 and fast. Experiments seem to show that these body- 

 cells carry the entire inheritance, though only certain 

 characters have found expression in them, the others 

 lying latent. In higher animals, where differentiation 

 has gone further, the regenerative capacity of a group 

 of body-cells is much restricted. Yet the cases we have 

 mentioned suggest that it is unwise to make too sharp 

 a contrast between body-cells and germ-cells. The 

 germ-cells are those cells which carry the whole inheri- 

 tance without allowing any of it to find expression until 

 appropriate conditions and stimuli are forthcoming. 

 They carry it in a form little liable to extrinsic influence 

 and yet readily admitting of development. 



4. Modes of Inheritance. Prediction of the result of 

 pairing two organisms is apt to be falsified for instance, 

 because the expression of a particular character in 

 development depends in part on other elements in the 

 inheritance, and also in part on the available "nurture," 

 such as the food, the temperature, the moisture, and so 

 on. But there are some well-known alternatives of ex- 

 pectation, and in some cases the average result may be 

 predicted with certainty. Let us consider the various 

 modes of inheritance. 



When similar forms are bred together for generations 

 and those furthest from the mean persistently removed, 

 a uniformity and constancy of character often results. 

 Two parents produce offspring like themselves and like 

 the rest of the herd. They are " pure-bred " and " breed 

 true." It is believed that the hereditary items (or 

 ' determinants," or " factors," or " genes ' for they 

 get many names) corresponding to a particular character 

 are present uniformly in all the germ-cells of both the 

 parents. Each has what all the rest have, none has what 

 the others have not. To parents and offspring alike the 

 technical term "homozygous" is applied. In such cases 

 there will be a strong hereditary resemblance and to the 



