1344 PHYSIOLOGY 



either to the individual or to the race. It has been suggested that 

 as soon as each individual concerned in the process receives the nuclear 

 material from organisms which may have been exposed to slightly 

 different circumstances, corresponding changes will be introduced into 

 the tendencies to growth of the product of the union. Some of these 

 tendencies may be more advantageous than before, while others may 

 be the reverse. Increased possibility of variation is, however, intro- 

 duced by this admixture of nuclear material, and this may be the 

 advantage of the process to the race. It should be noted that the half 

 of the nucleus lost by each conjugating organism is qualitatively 

 different from that which it retains and probably from that which it 

 receives. A generation in which the nucleus can be represented by 



ab, and which by simple division will produce similar organisms with 

 nucleus ab, conjugates with an organism of slightly different structure, 

 and therefore with a nucleus which can be represented as cd. After 

 conjugation, the db generation will contain a nucleus represented by 



ac, while the cd generation will contain a nucleus represented by bd. 

 ac or ltd may be better or worse combinations than ab or cd. If either 

 of them is better, that organism will survive under the less favourable 

 conditions, and the race will continue with a slight, and to us inappre- 

 ciable, change of type. 



REPRODUCTION IN THE METAZOA 



The numberless cells forming the bodies of the higher animals are 

 all produced by a series of divisions from a single cell, the fertilised 

 ovum. This cell is the result of a process of conjugation between two 

 cells derived from different individuals. With the multiplication of 

 cells forming a single organism there is of course an increased size of 

 the organism. It is doubtful whether this of itself would be of any 

 advantage, were it not that the multiplication of cells goes hand in 

 hand with differentiation, groups of cells being modified structurally 

 and set aside for one or other function of the body. Differentiation 

 of function implies higher functional capacity. As a motor organ or 

 as a means of locomotion, the differentiated muscle-cells, with their 

 attached parts, must be more effective than the undifferentiated 

 protoplasm of the amoeba. Specialisation of function involves changes 

 of type in the cells resulting from the division of the primitive 

 undifferentiated ovum. In most cases this change of type is perma- 

 nent. An epithelial cell such as that forming the epidermis or 

 the liver, when it divides, produces another cell of the same kind. 

 One might almost speak of the evolution of a new species of cell but 

 that it takes place within the short period of the development of the 

 multicellular individual, instead of occupying a long space of time, 

 and involving the destruction of countless individuals, as is the case 



