l6 MARINE BIOLOGICAL LABORATORY. 



dividing it more or less equally among many cells, 

 scattered or grouped according to the nature of the 

 work. Nature is provident as well as bounteous, and 

 so she determines the number of workers not only with 

 reference to ordinary needs, but also with a view to 

 emergencies. The cell cannot work on indefinitely. 

 Exhaustion follows exertion ; rest and recuperation are 

 as necessary to the cell as to an individual ; hence the 

 need of relays. The cell has its own term of existence, 

 which is usually much shorter than that of the organ- 

 ism ; hence the need of substitutes. So each class of 

 specialized cells may greatly exceed in number the actual 

 needs of the moment. One of the best illustrations of 

 this fact is seen in the reproductive cells, which are 

 often so enormously in excess of use, that they are 

 scattered in the water or the wind, with not one chance 

 in a thousand of ever fulfilling the purpose of their 

 existence. All such profuseness, however, has its mean- 

 ing, even though it only neutralize accident, and so 

 insure a few the realization of their proper destiny. 



But these hosts of cells suffice for only one of the 

 many varieties of reproductive work. They are called 

 reproductive cells, not to indicate monopoly of the 

 entire work, but pre-eminence merely in one important 

 branch of it. Propagation of the species is their task ; 

 but this becomes a monopoly only among the higher 

 forms of life. The same work may be accomplished by 

 budding and fission, processes which prevail very largely 

 among plants and many of the lower animals, usually 

 supplemented, however, by the more general process of 

 reproduction by means of specialized cells. 



But the generation of the species, which follows such 



