294 Dynamic Theory. 



to a vertical line, permitting thus, when least dilated, a full range of vision direc- 



tion in which these animals chiefly watch for prey." Observe that the only important 

 anatomical distinction named is one that crosses the lines of species, and depends on a 

 habit of the environment common to several species. Tbe same difficulty is found in 

 settling the boundary between the saurian ( lizzard-like) and ophidian (snake-like) rep- 

 tiles. A part of Cuvier's sixth family of saurians grade into the first family of ophidians. 

 And they also grade backwards into the third family of saurians the Iguanas. So that 

 from the Iguanas (lizzards) to the Orvets (snakes) is an uninterrupted series of graded 

 transitions. (Cuvier's Animal Kingdom, 267.) 



The difficulties of classification are so considerable in many cases that naturalists often 

 disagree as to the species and even to the genus of an animal. The species and genera 

 of mollusks, in particular, run into each other in the most perplexing manner. The 

 same thing is true of plajits. The species may be crossed, and are crossed in many ways, 

 and have been and continue to be modified constantly by new conditions of soil, climate, 

 cultivation, &c. 



CHAPTER XXXIII. 



ORIGIN OF SEX. 



Herbert Spencer has pointed out that in the growth of a cell the neces- 

 sary nutrition, respiration and excretion must be through the cell walls, 

 and that the surface of the cell wall is proportionately less in a large 

 cell than in a small one. The diameter of a sphere being one, its surface 

 will be 3. 14, and its cubical contents 0. 52, but if the diameter be 2, its 

 surface will be 12. 56 and its contents 4. 18; that is, by doubling the diam- 

 eter, the surface is increased four fold and its contents eight fold. At 

 this rate the mass of the cell will soon become too great to get its nourish- 

 ment through the surface, and when that point is reached, growth must 

 stop or greater surface must be obtained. It is obtained by the division 

 of the cell into two which is reproduction by fission. Where the cells 

 after such division remain attached to each other, so as to form a com- 

 posite body, as in plants, the increase of the body is growth; but where 

 cells, after division, separate from each other and each piece goes on 

 growing up to the mature size of the parent cell, it is growth to be sure, 

 but it is interrupted and disconnected growth, or, as it is usually ex- 

 pressed, discontinuous growth. But it is also reproduction, for it repro- 

 duces the parent cells and perpetuates the race. This form of repro- 

 duction is called asexual. The lower forms of vegetation reproduce asex- 

 ually by detachable buds or bulbils, or by fission, as some algae, the 

 liverworts, many ferns, some grasses, &c. Animals which reproduce 

 by budding, or gemmation, or by fission, are asexual. Most protozoa, such 

 as the Gregarinae, Monera, Amoebae, some Sponges, &c. , many Coelenter- 

 ates, as hydra, corals, sea anemones, and some worms like certain Turb- 

 ellarians and sea worms, liver flukes, tape worms, &c. , reproduce asex- 

 ually. The growing of the different parts of an animal or a vegetable 

 consists of the division of the cells in the same manner as the asexual 

 reproduction of the protozoa. The cells, when so produced, however, 



