1 6 DARWINISM AND EVOLUTION 



a seed is produced. Now there is nothing more certain than this, 

 that a seed contains vast forces in a state of tension. Supply the 

 seed with warmth and moisture, and these forces are liberated, 

 and reproduction completes itself. 



We witness the same series of events in animals. When the 

 body has ceased to grow, but when the quantity of food taken 

 and assimilated continues to be much the same as before, the 

 force produced, unable to expend itself in growth, enters a state 

 of tension, becomes latent, potential, and results in the repro- 

 duction of the species. 



Having grasped this idea, let us, in order to understand the 

 reason of the sterility of hybrids, take a very simple illustration. 

 Let us suppose the period of the tensional molecular vibrations 

 of the horse to be 20 ; and, as a matter of course, let us suppose 

 that the period of the molecular vibrations of the mare is also 20. 

 These periods will reinforce one another, and fertility results. 

 Now let 19 be the molecular period of the ass; 19 and 19 will 

 also reinforce one another. But in the case of a mule, when 

 the ass is crossed by the horse, we have molecular periods 

 of 19 and 20, which not only will not fully reinforce one another, 

 but will occasion some amount of what is called " interference," 

 which may be more or less according to circumstances. And the 

 result is that the tensional molecular vibrations of the mule 

 having suffered interference, they are no longer able to reinforce 

 one another, and the hybrid is sterile. The total sterility between 

 some animals would in like manner be due to the fact that their 

 molecular periodicities, say 7 to 131, are such as afford each other 

 no reinforcement whatever. On the other hand, in " breeding 

 in and in," as consanguineous crossing is called, the molecular 

 periodicities are so exactly alike that even special tendencies, like 

 a tendency to disease, are reinforced ; while in exsanguineous 

 crossings there is sufficient " interference " in the molecular 

 periodicities to make such tendencies cancel out. 



At length we have reached Prof. Fiske's last proposition, which 

 is the postulate that since the first appearance of" life upon the 

 earth's surface, sufficient time has elapsed to have enabled such 

 causes as the foregoing to produce all the specific heterogeneity 

 now witnessed. The time which has elapsed we have no means 

 as yet of accurately measuring. Perhaps our best index is a 

 consideration of the formation and destruction of stratified rocks, 

 of deposition and denudation. 



Shells of dead animals have lain for long on the ocean's floor 

 before they were embedded in sediment. Fossil oysters in clay 

 are found with serpulse, barnacles, or corals attached to the inside 

 of the valves. Often, too, the creatures that have attached them- 

 selves to the empty shell of a mollusc have not only grown to 

 maturity before the shell was covered with deposited matter, but 



