2/O GEOLOGICAL BIOLOGY. 



ism, or a pair of them in each specific case, from which all 

 the other representatives of the genus spring by natural gen- 

 eration without change which is the old creational theory of 

 origin of species ; but by the individual assuming a different 

 course or extent of ontogenetic growth from the course or extent 

 of growth of its ancestors, including acceleration in the growth 

 of a part, or of an organ, with increase or specialization of its 

 function. 



Intrinsic and Extrinsic Development, and Intrinsic and Extrin- 

 sic Characters. This brings us to the consideration of the 

 twofold nature of the morphological and physiological char- 

 acters possessed by organisms. There are two fundamen- 

 tally different ways in which we recognize the characters as 

 differing one from the other when looked at from the evolu- 

 tional point of view. When we mark the course of develop- 

 ment from the egg to the adult chick we observe that there is 

 a gradual building up, first of tissues, then of definite organs 

 made of those tissues, from simple uniform cells; or, going 

 further back, from the original nucleated, unsegmented cell 

 itself. This is a process of differentiation of parts, as has 

 been already defined, and with specialization of functions. 

 But it is a process of the increase of parts and functions by 

 division of labor, and is an expression of one of the funda- 

 mental laws of the organism as a whole. This kind of growth 

 we may call intrinsic development : intrinsic, because it has 

 to do with the expansion or development of the organism as a 

 whole, and involves the internal adjustment of the organism 

 itself, and not simply the modification of one of its parts. 

 There is another kind of elaboration of organs and functions 

 which consists in the multiplication of like parts performing like 

 functions, and results in difference in the size, the proportion, 

 or the number of the morphological parts. This kind of growth 

 we may call extrinsic development, because it appears to be 

 definitely correlated with the nature and amount of the ex- 

 ternal supply of materials for growth, and with the outward 

 demands upon the activity of the functions concerned. 



The distinction thus established in the mode of origin of 

 the characters furnishes the basis for the classification of the 

 characters into intrinsic and extrinsic characters. 



