DARWINISM AND EVOLUTION. 1 5 



may, must, and do undergo such correlated variations as to 

 become at last mutually infertile. 



And here comes up the interesting subject of the sterility of 

 hybrids, and we may give it a brief consideration, though it has no 

 direct bearing on Darwinism or Evolution. 



Complete sterility is termed Agenesis, and complete fertility, 

 Eugenesis. Examples of Eugenesic crossings, as given by Broca, 

 are those between dogs and wolves, goats and sheep, camels and 

 dromedaries, hares and rabbits. 



But within the extremes of sterility and fertility are the two 

 conditions of Dysgenesis, as between the horse and the ass, where 

 mules are sterile ; and Paragenesis, as between Negroes and 

 Teutonic Europeans, where mongrels of the first generation have 

 only a partial fecundity, though they breed more readily with the 

 parent species. Similarly, in plants, the hybrids of intermediate 

 types between the two parent stocks are sterile, while those resembling 

 one or the other species are prolific. 



Let us consider the problem in terms of force. Force is what- 

 ever moves matter, or tends to move it. In the first case we have 

 motion ; in the second case we have tension. Accordingly, the 

 so-called vital forces, or the forces which an organism derives from 

 its food, may produce either motion or tension. There are two 

 forms of motion : molar, the motion of masses ; and molecular, 

 the motion of atoms. While the first form consists chiefly of 

 locomotion, or movement from place to place, the second form 

 consists chiefly of vibration, which is movement in one place. 



As a plant grows we see matter in motion. Within limits, the 

 more food a plant has the more it grows ; the more stems and the 

 more leaves it puts forth. It is well known that flowers, which are 

 the reproductive organs of plants, are leaves without stems. The 

 sepals, petals, stamens, and pistils are all the homologues of 

 leaves, and these, without internodes, or intermediate stems, are 

 arranged in whorls or spirals. Flowering, then, is, in a sense, the 

 opposite of growth. Instead of there being a long internode 

 between each floral element, these elements are all crowded down 

 together, and the energies of the plant are expended, not in 

 motion, but in tension. 



Now, molecular agitations have a numerical period, and 

 vibrations of like periods reinforce one another. Hence the 

 molecular agitations of the ovule are able to be reinforced by 

 those of the pollen of the same species, because they are both of 

 the same numerical period of vibrations. 



In some lowly organisms, the union of two elements is 

 unnecessary to reproduction ; but as evolution advances, as the 

 structure to be ultimately built up becomes more and more 

 complex, reinforcement is required. When the tensional energies 

 of the ovule are reinforced by the tensional energies of the pollen, 



