ox COAL. 165 



families or classes, and stood as their representative until these fami- 

 lies or classes were separately introduced. The Placoids and Ganoids, 

 for instance, stood during almost the whole palfeozoic period the sole 

 representatives of the vertebrate type, combining in themselves the 

 characters of all classes, and thus prophesying their coming, until 

 Nature was fully prepared for their introduction. The Sigillaria and 

 Lepidodendron stood as the representatives of both Cryptogam and 

 Phoenogam, until these two ideas were separately and more distinctly 

 expressed by the subsequent introduction of the typical forms of these 

 two classes. It is as if Nature first sketched out her work in general 

 terms and then elaborated each subordinate idea in separate families ; 

 all these families, taken together as an organic whole, still containing 

 the original idea in a more completely developed form, as if the pro- 

 blem of organic nature was first expressed in a few simple but com- 

 prehensive symbols and then differentiated. Organic nature has 

 often been compared to a broken chain, the disjointed links of which 

 are the widely separated and distinctly marked families of tlie present 

 fauna and flora, and the connecting links of which are to be found 

 deep buried in the strata of the earth. But the complexity, the 

 beauty, and, more than all, the life, growth, and development of Na- 

 ture, is not to be represented by any such miserable mechanical con- 

 trivance as a chain. It is rather a tree — a tree of life — a tree whose 

 trunk is deeply rooted in the lowest paleozoic strata_, whose first 

 giant arms are given ofi" in the carboniferous, which branch again in 

 the secondary and again in the tertiary periods, while its extreme 

 branchlets, and also its flower and fruit, are the fauna and flora of the 

 present epoch. The object of geology is to trace each branch to its 

 fellow branch, and each limb to its fellow limb, and thus gradually to 

 restore the whole noble form and determine the laws of its growth. 



This differentiation, this passing from simplicity to complexity, 

 from unity through diversity to a higher unity, is the fundamental 

 law of development. Let me illustrate my meaning by a few simple 

 examples : Tbe ultimate anatomical elemeurs of every organized body, 

 whether animal or vegetable, are cells. The whole body is made up 

 of cells, and all the bodily functions are performed by cells. In fact, 

 the body may be looked upon as an organized community of indi- 

 vidual cells. Now, if we trace these cells from their earliest condition 

 in the embryo to their mature condition in the fully developed animal 

 or plant, or from the lowest animal or plant regularly to the top of 

 the scale, we will observe a most beautiful instance of the difteren- 

 tiation of which I speak. The cells are at first all alike, simple and 

 globular, and each performs all the functions appertaining to cells, 

 though comparatively imperfectly. But as development advances the 

 cells begin to take on different forms and to perform different func- 

 tions. Some become nervous cells, some muscular cells, some biliary 

 cells, &c., until, in the mature condition and in the highest animals, 

 the diversity of form and specialization of function reaches the highest 

 point, each form of cell being confined to the performance of a single 

 function. 



If, instead of the ultimate anatomical elements, the cells, we take 

 the proximate anatomical elements, the organs, or even the regions of 



