The number of deaths being due to the number and activity of enemies and the severity of the environment, it follows that when no care can be given to the offspring the birth rate is strictly proportional to the severity of the struggle for existence. If the enemies are very slowly increased then there is a greater survival of the offspring of those having the largest families, and a progressive increase in the birth rate, from the extinction of those with the least number of offspring. Thus, certain sea birds have so few enemies that they survive though they lay but one egg a year at their nesting homes on the barren rocks in mid-ocean. On the other hand, the increasing enemies of the codfish gradually killed off the descendants of those having a small number of offspring, until now the cod lays many thousand eggs a year, and no matter how many hatch out, all but two of them, on an average, are eventually killed before leaving descendants. The number of offspring is also inversely proportional to the protection they receive from their parents. The lower animals, giving them no care whatever, allow the enemies full play, but this is really only one element in the problem.
Certain ground birds must have twenty or thirty hatched eggs a year, even if they do not give the young the greatest care, far greater than the sea birds having but one egg a year.
If the enemies increase suddenly, not allowing for adjustment to the environment by increase of offspring or increased ability to escape the enemies, then extinction results as in the case of our buffalo. If the enemies are lessened as by changing the environment then the animal increases prodigiously. The mongoose in India has six litters a year, of five to ten offspring each time, but its enemies kill them all except two, and can then kill the parents also, yet the mongoose runs no risk of extinction. Transported to Jamaica to kill rats, it has overrun the Island and bid fair at one time to ruin all the plantations. There is the similar rabbit pest of New Zealand and Australia, a pest which also ruined one of the Madeiras* about 1435.
Evolution cannot take place unless there are adversities to kill off those whose variations make them the least fit, so that there will be a survival of those better fitted to the environment, and a gradual change in the species. Under no other conditions can change take place, for if all are preserved there cannot be a survival of the fittest. We need not go into an explanation of why there are no two individuals exactly alike in any species, how all vary from the average, and how some of these variations make the individual better fitted for the environment than his brothers or cousins and more likely to survive and leave offspring. What is necessary for the present discussion is to realize that there is always a ruthless destruction of life in the struggle for existence when evolution occurs, and that this struggle depends upon overpopulation.
The average of a species must necessarily be out of adjustment to its environment if evolution occurs, because it is those varying from the average which survive. If the average were the best fitted then they would survive in the largest number and there would be no change in the species, a condition existing among a few marine animals, which after millions of years are the same as their paleozoic ancestors.
* Porto Santo.
We can apply all these rules to man, for he always exists in as dense masses as he can. He has advanced the most of all, has always been out of adjustment, and those better fitted than the average are the survivors. Death of excessive numbers is the price paid for the advance of the survivors. The tendency to spread is, then, a natural phenomenon wholly beyond our control. The courtiers who thought that the Norse king could control the sea waves, and Xerxes who whipped the Hellespont, were not more foolish than modern men who think that by a word we might control the spreading waves of population. It is common knowledge that the ocean waves are under the guidance of perfectly definite and rigid natural laws, and that their speed, size and power can be calculated almost as accurately as eclipses of the moon. There are spreading waves of every species of living thing, man included, and they are as rigidly controlled by definite laws as are the currents of water. Though the laws relative to the movements of population have actually little in common with the laws of fluids, yet the analogy between the two is remarkable. A fluid is a mass composed of particles which move about freely among themselves, the higher the temperature the more rapid and the greater are the excursions of each particle. Population is likewise a mass composed of units which move about freely among themselves, and the greater the excitement the greater and more rapid the movements.
Population sometimes flows sluggishly like lava, as in the gradual spread of Teutonic races into America, sometimes fiercely like volatile ether, as in those frightful excursions of the Mongols into Europe. In the former case there is adhesion to the surface, as with oils; in the latter there was separation from the surface, as with fluid mercury.
The path selected is that of least resistance. Though most rivers are now flowing as they have for untold thousands of years, yet they are constantly deviating in obedience to new forces, and though the channels of human travel are virtually the same on land as they have been since prehistoric times, yet there are constant deviations in obedience to new forces, such as the steam engine, which make new paths of least resistance.