GEOMETRICAL MODES OF DEVELOPMENT. 457 



fact that it passes through a definite series of changes. Commencing at 

 first as a simple cell, to which what might be termed a momentum of de- 

 velopment has been imparted, it assumes one form after another in suc- 

 cession, but is ever ready, like the moving bodies of mechanics, to obey 

 the impulses which extraneous forces may impress upon it. Properly 

 speaking, we can never say of an organized being that it is in a condi- 

 tion of rest. In truth, it is always in motion. It has a past and a fu- 

 ture coming from one state and going to another ; and though, to use 

 the language of mechanics, the inertia that it has at any moment must 

 tend to continue it in the state at which it is then found, since it varies 

 by degrees from one condition to another, we are obliged to look upon it 

 in these variations just as we should upon an inorganic mass under sim- 

 ilar circumstances, and, guided by the incontrovertible law of physics, 

 that (jyery change of motion is to be attributed to the influence of a force, 

 we must impute its passage from state to state to the intervention of a 

 like agency. In this respect, the career of an organic combination, in its 

 two conditions of maintaining for a time a similarity or passing through 

 metamorphoses, presents a general analogy to the uniform rectilinear, and 

 to the varied motion of mechanics. 



As we have just remarked, the most elementary organic combination 

 appears to be a simple cell. This, under circumstances Tllree 

 which we shall presently consider, may pass into develop- rical modes of 

 ment by multiplication in three different ways, geometrical- 

 ly distinct. Its development may be in one, two, or three dimensions 

 linear, superficial, or solid. As illustrations may be offered the proto- 

 coccus, which is a simple cell ; the linear conferva?, consisting of a row 

 of cells which perpetually undergo terminal extension, the line becoming 

 longer and longer as development of new cells at the end goes on ; the 

 ulvas, in which increase takes place simultaneously in length and breadth ; 

 and any of the higher forms, which grow simultaneously in length, 

 breadth, and thickness. Whatever the manner of development may be, 

 or whatever the condition presented as the combination passes from 

 phase to phase, no doubt can be entertained that it takes place in conse- 

 quence of the agency of forces which are acting under definite laws ; and 

 though, even in the case of organisms low in the series, a geometrical 

 definition of their form is impossible, this is because of the imperfection 

 of our knowledge, and is no kind of indication that there has been any 

 irregularity or wantonness of play in the forces at work. 



Asserting thus in the broadest manner the influence of physical forces 

 over development, and seeing that dynamical physiology must i nqu i ry into 

 be committed to those conditions, and those alone, which are the existence 

 universally recognized in positive science, I shall proceed, in priLlpJe'of 

 this chapter, to set forth the views we entertain respecting the r s aniz ation. 



