544 MANUAL OF ELEMENTARY ZOOLOGY 



its own production, and the most that we can say is that 

 there is a possibility that the same principle is involved 

 here as in assimilation. In the light, however, of our 

 present knowledge, we must regard assimilation as a 

 property peculiar to living beings. 



5. In regard to the processes which constitute re- 

 production it is again impossible to draw a parallel between 

 living and lifeless things, (a) The case of fission is simpler 

 than that of development. Drops of many colloidal solu- 

 tions will in certain circumstances break up into droplets 

 in a way that suggests the fission of those lower organisms 

 whose whole body breaks up in reproduction into two or 

 more apparently similar parts ; and it is claimed, but dis- 

 puted, that the process is the same in the two cases. 

 We need not search, however, for analogies to fission among 

 lifeless things. It is probably closely related to contrac- 

 tion, whose claim to be peculiar to the living body we have 

 already seen to be as yet undecided, (&) One part of 

 the problem of development may be dismissed in the 

 same way as that of fission. The series of changes by which 

 the adult is developed from the germ are but further 

 instances of phenomena we have already discussed, such 

 as growth and chemical change, and so far there is nothing 

 new to us in development. But a complication is here 

 introduced by the fact that the processes by which the 

 germ develops take place in such a way as to produce for 

 the offspring a body of the same kind as that of the parent. 

 We have seen that the property to which this is due is 

 called heredity. Now in heredity we do meet with a 

 peculiarity of living things, though it is perhaps not 

 immediately obvious wherein this peculiarity consists, 

 (i) Heredity is not peculiar in that it enables a part of 

 the body which is broken off to retain the properties of 

 the mass from which it is broken. This is equally the 

 property of any material body — as, for instance, of the 

 droplets to which we have alluded, (ii) It does appear at 

 first to be peculiar in that it enables a portion of matter to 

 revert by its own activity to a structure which it formerly 

 possessed. But the fact has been pointed out that some- 

 thing like this happens in various . kinds of lifeless matter. 

 A mass of plastic sulphur, for instance, will in time revert 



