THE CATEGORIES OF LIFE 



IT is probable that there are at least about 

 500,000 distinct species of plants and animals in the 

 sea. Each species is an assemblage of individuals 

 which differ from the individuals composing other 

 species or assemblages, in form, in life-history, and 

 to some extent in habits. Above all, the individuals 

 of each species differ from those of other species in 

 that their mode of reproduction has been so special- 

 ised, that the offspring born from them resemble the 

 parents more than they resemble the individuals of 

 any other species. Now it is the task as yet an 

 incompletely performed one of marine biology to 

 classify all these organisms so as to display their inter- 

 relationships to each other and to the species of 

 organisms which have inhabited the earth in past 

 geological periods. Even to indicate the main prin- 

 ciples of such classification would be beyond the scope 

 of this book, but if we assume a general knowledge 

 of the main types of plant and animal life it will not 



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