44 



OUTLINES OF EVOLUTIONARY BIOLOGY 



another. As a result of this specialization the various functions 



may be much more efficiently and economically performed, but a 



no less important result will be that the different individuals will 



i no longer be able to lead independent lives if separated from 



one another they will perish because unable to perform for 



' themselves individually all the functions which are necessary for 



their existence. 



In colonies of Protista we meet with little if any of this 

 differentiation and division of labour ; from the physiological 

 point of view the cell units remain almost if not quite inde- 

 pendent of one another, and that is why they are still classed 



amongst the unicellular organ- 

 isms. There can be no doubt, 

 however, that it was this habit 

 of colony-formation that led to 

 the origin of true multicellular 

 animals and plants Metazoa 

 and Metaphyta from unicel- 

 lular ancestors. The compo- 

 nent cells of a colony gradually 

 became integrated to form an 

 individual of a higher order, 

 and this process was accom- 

 panied by that differentiation 

 and division of labour which 

 in course of time led to the 

 astonishing complexity of 

 structure which characterizes 

 the higher members of both the animal and vegetable kingdoms, 

 In contrast to this complexity of the organism as a whole we 

 shall find that the individual cells of one of the higher animals 

 or plants are usually much simpler in structure than the more 

 highly organized Protista. After what we have said about 

 differentiation and division of labour the reason for this should 

 be sufficiently obvious. 



Perhaps no more convincing demonstration of the applicability 

 of the cell theory to the higher plants and animals could be 

 given than that which is afforded by the study of development, 

 for, however highly organized a plant or an animal may be in the 

 adult condition, it always commences its individual existence as 

 a single cell the fertilized ovum or zygote and attains the 



FIG. 12. A colony of Sphserozoum, 

 cut in half. (After Haeckel, in 

 " Challenger " Eeport.) 



