186 THE POPULAR SCIENCE MONTHLY. 



has continued in the march of development, while the career of the 

 other has been stopped at an earlier point. The organic aspect, at 

 last assumed, is the representation of the physical agencies that have 

 been at work the environment. Had these for any reason varied, 

 that variation would at once have been expressed in the resulting 

 form, which is, therefore, actually a geometrical embodiment of the 

 antecedent physical conditions. For what reason is an offspring like 

 its parent, except that it has been exposed during development to the 

 same conditions as was its parent. Comparative physiology is not a 

 fortuitous collection of experiments. Our noblest conception of it is 

 the conception we have of analytical geometry. Each member of the 

 organic series is an embodied result of a discussion of the equation of 

 life for one special case. 



It was a felicitous thought of Descartes that we may represent a 

 geometrical form in an algebraic equation, and, by the proper consid- 

 eration and discussion of such an expression, determine and delineate 

 all the peculiarities of such a form ; that here it should become con- 

 cave, there convex ; here it should run out to infinity, there have a 

 cusp. The equation determines all the peculiarities of the form, and 

 enables us to construct it. In like manner, all living and lifeless 

 forms are related ; an increase in the value of one condition carries de- 

 velopment forward in one direction ; an increase in the value of an- 

 other condition determines development in another way, and these 

 variations give rise in their succession to the whole organic series. 



Nature ever geometrizes and ever materializes. Every organism 

 is the result of the development of a vesicle, under given conclusions, 

 carried out into material execution. It is the incarnation, the embodi- 

 ment, the lasting register of physical influences, the daughter of the 

 environment. 



Let us now rapidly survey the changes that have taken place in 

 the earth's organisms : 



In the earliest, or Primordial period, there existed of plants only 

 water-organisms tangled sea-weeds. Then in the following, the Pri- 

 mary, came the more perfect cryptogams, such as ferns. Then fol- 

 lowed, in the Secondary, pine-forests. In the Coal period the phane- 

 rogamia developed out of the more perfect cryptogamia. Not until 

 the Chalk did the higher corolliflorae appear. In the beginning of the 

 Tertiary the earth had sufficiently cooled at the poles, climate-zones 

 were produced, and the land was covered with leaved forests. Flow- 

 erless plants had been succeeded by flowering ones, the latter first 

 without a distinct corolla, and then by those with one ; and of these, 

 first the lower and then the higher. 



Turning to the order of succession of animal life of the Pri- 

 mordial, the forms are skull-less ; then in the following, the Primary, 

 came fishes, first those with the heterocercal tail, as in the embryos of 



