l68 GEOLOGICAL BIOLOGY. 



Secondly. Some sort of division or fission sets in which 

 begins with the cell-nucleus; if fission becomes complete, it 

 is unicellular reproduction and the organism is protozoal. 

 This process, repeated over and over again, is what may be 

 called cell-genesis, or cryptogenesis. This is unspecialized 

 growth, and the cell, when considered as carrying on inde- 

 pendent existence, may be called an undifferentiated organism. 



Thirdly. When the fission of the developing cell is incom- 

 plete within the walls of the cell, the process goes on until 

 there is repeated cell-division, or segmentation, and the 

 dependent cells are more or less specialized and combine to 

 form tissues. 



Fourthly. The tissues develop into separate organs, capa- 

 ble of carrying on special functions, and we have a metazoal 

 animal, in which the several parts act for the interest of the 

 whole body. The product is a complex organism with organs 

 made of specialized cells performing special functions. 



Growth, strictly speaking, is thus a function of the cell, 

 which culminates in cell-multiplication by fission, or partial 

 fission, augmenting the mass and force of the individual. 



Development is that kind of growth which takes place in a 

 multicellular organism when, by generation, a nucleated cell 

 is set apart, protected, nourished, and by division and differ- 

 entiation is elaborated into a complex organism, without 

 regard to the growth of the parent — even at its expense, and 

 when fully constructed is set free to begin independent life 

 for itself. 



Evolution is that kind of growth which is expressed in the 

 specialization of functions and differentiation of organic struc- 

 ture in some members of a species, enabling them to exceed 

 the capacity of their ancestors, and to adapt themselves to 

 conditions beyond or other than those to which the parent 

 form was adapted. The evolution is exhibited in a series of 

 forms, succeeding one another, in which varietal, and ultimately 

 specific, differences distinguish the later from the earher mem- 

 bers of the series. Such a series is called a race, and the repre- 

 sentatives of a race which are alike are called a species. 



Embryology. — The development of the individual is par- 

 ticularly discussed under the name of Embryology, and the 



