40 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



at all speedily. It was millions of years after the first 

 many-celled animals appeared on the earth before the first 

 insect appeared. And still millions of years later before the 

 first backboned animal was evolved. 



As to the step from isolated single cell to united many cells, 

 it was undoubtedly made in the simple way still represented 

 by a few living organisms known as Volvocinae, sometimes 

 called plants, and sometimes animals. These are one-celled 

 organisms that live as small colonies or groups of cells. These 

 few cells, only sixteen in the case of several of these organisms, 

 are all derived from a single cell by its division into two and the 

 succeeding divisions of these two into four, the four into eight 

 and the eight into sixteen. And they are all alike. They 

 remain together in the form of a tiny ball, the cells all imbedded 

 in a soft gelatinous substance secreted by them. Each cell has 

 a pair of flagella, and the waving of all the flagella moves the 

 little ball through the water. Each cell can take up food, 

 respond to stimuli, and in fact do all the things that we have 

 found are essential to living and which are done in the simplest 

 manner by the one-celled animals. Indeed it is probable that 

 each cell could live independently; and as a matter of fact each 

 one does for a short time when the colony breaks up after reach- 

 ing maturity. 



For when this little colony is mature and ready to repro- 

 duce itself, the gelatinous stuff dissolves, the sixteen cells 

 are set free in the water, and each, by repeated division may 

 produce a new colony. Or a process of conjugation between 

 pairs of the freed cells can take place, and from each paired 

 cell formed by the conjugation of the two, a new colony may 

 be formed by simple division. 



Differentiation and Specialization of Cells. If this first step 

 toward making a many-celled animal out of a single-celled one 

 seems simple, the next step does not. In the simplest kinds of 

 true many-celled animals an important new condition appears. 

 It is a condition of differentiation or specialization of the cells 

 united to form the body. The cells are no longer all alike in 

 appearance, and no longer have identical capacities. Only a 

 few of them remain in simple generalized condition. These 



