182 MANUAL OF ELEMENTARY ZOOLOGY 



its structure is essentially that of a polyp, we might 

 assume that each polyp is also an individual. (2) On 

 the other hand, the whole polyp stock is a unit, and we 

 might consider it to be one individual, of which the separate 

 polyps are members, still regarding the medusa as an 

 individual. (3) From this we might go further, and claim 

 that, as the medusa is morphologically equivalent to one 

 polyp head, it is but a member of the individual to which 

 the polyps belong, though for purposes of distribution it 

 separates from them, and thus we might regard as one dis- 

 joined individual the whole mass of forms which arise from 

 the fertilised ovum which gave rise to the polyp stock. 



These alternations must now be examined, (a) We may 

 begin with the third of them. This may be stated, for 

 wider use, as follows : " An individual in Biology is the 

 whole of the living matter which is formed by a zygote 

 before the gametes to which it gives rise have themselves 

 conjugated." Besides its morphological basis, which we 

 have just seen, it is founded upon the belief that the life 

 of every kind of protoplasm is divided by conjugation into 

 periods, at the end of each of which it makes a new start 

 which can be made in no other way. The grounds on which 

 this theory is based are twofold — (i) a theory that all proto- 

 plasm becomes at length senile and can only be rejuvenated 

 by conjugation ; (ii) a theory that heritable variations only 

 occur as a result of some recombination of substances 

 which takes place in conjugation. If these theories were 

 true, a zygote, and only a zygote, would make a new begin- 

 ning, both in regard to vigour, and in that it had in itself 

 permanent variation from each of its parents. In the 

 next chapter, however, we shall see that conjugation 

 cannot be held to have per se a rejuvenating effect on 

 protoplasm. And on the other hand, though most of the 

 variations which can be inherited do undoubtedly arise 

 at conjugation, it cannot be the case that all do so, for 

 otherwise organisms which are continually self-fertilised 

 could never vary, nor would the remarkable phenomena 

 known as " bud-sports " in plants be possible. Moreover, 

 if we adopt this view that all the products of a zygote are 

 a single individual, we are compelled to hold, not only 

 that all the vast host of living beings which arise by the 



