22 GENERAL EVOLUTION. 



these results would appear are, of course, unknown. The me- 

 chanical arrangement and condition of the parts would have 

 much to do with it. 



We are, however, here in the arcana of life, and the forms of 

 law which rise before the mental vision are but as the statues of 

 Memnon that greet the toiler on the river of mysteries ; they loom 

 upon its banks in twilight,, and the when, the how, and the where- 

 fore remain unanswered. But the river of Africa is yielding, her 

 secrets, and, though the life that she nourishes may be the last she 

 will give up, it is no less surely promised to the patient inquirer. 



A great advance toward an explanation of the operation of this 

 growth-force was made in the demonstration of the fact that its 

 highest exhibitions are confined to the multiplication of cells by 

 division of pre-existent ones, by contraction of their nuclei and 

 walls in lines which finally meet. 



The construction of additional parts consists solely of this cell- 

 growth, but the character of the result is, of course, dependent on 

 the position at which this addition takes place. It may be at the 

 terminus of a limb to add another toe, or on the wall of an artery 

 to add a valve. It may be in the brain to add a band of fibers, 

 or on the edge of a muscle to extend its width. 



That tissues are made of cells of original or altered shape, sepa- 

 rate or confluent (flown together) is well known. That the ar- 

 rangement of tissues into organs is due to the direction of this 

 multiplication is also true. Thus a gland is a collection of folli- 

 cles, each of which is a bagging of a plane tissue. This bagging 

 is an exaggerated convexity, which is occasioned mechanically by 

 excess of cell-growth at one spot on a uniform surface. Solid 

 parts are all formed, in the first i7istance, of simple segments. 

 These are parts produced in a straight rod by excessive elongation 

 or growth ; the process being as before, the division of cells and 

 distribution of homogeneous protein between them. All this is 

 derived from the study of homologies combined with embryology, 

 and the result is wonderful, and simplifies at once our ideas of the 

 action of the growth-force. It is mainly a repetition of cell-divis- 

 ion, the result as to structure being entirely dependent on the 

 influences which locate its activity and regulate its amount. 



Now, its amount will depend on the capacity of the existing 

 organism to convert heat, etc., into it ; and the form, as to tissue, 

 etc., in which it appears, depends no doubt on the complexity of 

 the machines or organs of which the organism is composed. In 



