152 INTRODUCTION TO GENERAL PHYSIOLOGY 



shall see presently. In the process of the division of nucleated 

 organisms, the nucleus plays an important part, undergoing a series 

 of complex changes by which it finally becomes two nuclei similar 

 to the original, one in each of the new cells (E., p. 226). 



We see thus how it is possible to speak of the " immortality " 

 of such organisms, since the substance of the parent does not 

 degenerate, but is divided up after increase in dimensions. 



This mere increase in size does not, of course, involve the acquire- 

 ment of any new capacities of adaptation to the surroundings. The 

 new material added is built up like that from which it grows. 

 Accordingly, we find, even in such unicellular organisms as the 

 protozoa and algae, that a further process has been developed, which 

 is the gradual beginning of the wonderful phenomena of sex, that 

 play so large a part in the beauty of the world and the brave deeds 

 done in it. 



We find, to begin with, that two similar individuals fuse together, 

 forming one single larger individual, which may then proceed to 

 divide in the old way, or it may split up into a number of smaller 

 individuals, each of which grows up to the normal size, and may 

 then continue to multiply in the simple way of division into two. 

 In either case the result is the union in the new generation of the 

 qualities of two individuals, which, however similar they may appear 

 to the eye, will almost invariably differ in their modes of reaction to 

 surrounding changes, as also in their capacity for " variation," as we 

 shall see later. This simple process of union of two individuals, in 

 which we cannot as yet speak of either of the pair as being male or 

 female, is known as " conjugation." 



It was supposed for some time that a race of unicellular 

 organisms, continuing to multiply merely by subdivision, sooner 

 or later died out, unless conjugation occurred. The race was 

 thought to undergo senile degeneration, and to be rejuvenated by 

 conjugation. But it is not clear how this was to be brought about, 

 otherwise than by the combination of the capacities of two different 

 individuals, and it has now been shown that, if a proper supply of 

 the materials necessary for growth is provided, there is no need for 

 conjugation, and the vigour of the race remains unimpaired. 



It will be clear that, since the advantage to be derived is the 

 combination in the new individual of different qualities, there is not 

 a great possibility of variety of experience in conjugation between 

 cells of similar situation and habit, especially when the organism is 

 not an actively motile one. The opportunity for conjugation occurs 

 only between cells that chance to be in proximity to one another 

 and have been exposed to closely similar conditions. The next 

 step, therefore, and one taken very early, is the differentiation of 

 the two gametes, as they are called, into two kinds of cells, one 



