The student may be interested to find how many- 

 there would be in that time, 

 f. Man. Man produces much more slowly than any 

 of the forms mentioned. Physically it would be 

 possible for the healthy aboriginal man to reproduce 

 about once in fifteen or eighteen months on an 

 average. Or during the possible reproductive 

 } period of about thirty years one pair might produce, 



say, twenty offspring. Before the last one was 

 '; produced, the oldest would be in its reproductive 



period, and thus the generations would overlap. As 

 a matter of fact, we do not find families of this size. 

 Marriages take place much later than the beginning 

 of the reproductive period. The lack of strength 

 ' on the part of civilized women, the trouble and 



cost of rearing large families of children, the 

 impossibility of rearing that many children wisely, 

 and many other such considerations make it out of 

 the question for human beings to reproduce up to 

 the animal limit. Parents are not willing to make 

 this much sacrifice, and it would not be wise if 

 they did. 

 In new countries the average rate of increase in a single 

 generation is between three and four children to the family. 

 In old countries it is less, going down close to two 

 children for each family, or in some countries a fraction 

 less than two. This means gradual "race suicide,'' for a 

 species can not be kept going if a pair brings less than a 

 pair to maturity. Let us assume the higher number for 

 the new country, or four children to each home. It is 

 clear at this rate that each generation doubles the native 

 population, and that if all the human beings lived, a very 

 few generations would stock the earth up to its utmost 

 limit of support. Some parts of the world, as the more 

 populous parts of China, seem to have reached this 

 condition now. 



4. The Diffierence between the Rate of Reproduction 

 and the Real Increase. Any one of the species of plants 

 and animals would become a pest to us if they really 

 multiplied at the high rates indicated as theoretically 

 possible. The human race itself would soon find life 

 intolerable if it reproduced to its full limit. But no species 

 ever does. The winters and the drouths, starvation and 

 internal weakness and disease, parasites and other enemies, 

 — such things sweep off countless millions of all these 

 species. It is a great struggle that goes on all the time 

 among all the animals and plants that are born, for, in the 

 long run, if a million are produced by two parents, only 

 two can come to maturity. Many students think that it 

 is out of this severe struggle that improvement has come 

 to plants and animals, for in the struggle the weakest and 

 poorest are destroyed and those survive that for some 

 reason or other are best adapted to all the important 

 factors in the environment. The rate of reproduction in 

 this way seems to be a matter of considerable value in 

 determining progress. 



CHAPTER TWENTY-FIVE. 



CROSSING BREEDS. 



1. Reproduction without Union. In reproduction such 

 as fission in bacteria, budding in yeast, spore production 



in molds, and stolons, buds, grafts, or tubers in the higher 

 plants, only one parent takes part in the formation of the 

 new individual. In such cases the offspring is likely to be 

 just like the single parent, because it is simply a portion 

 of it. The single parent stamps its nature most strongly 

 upon the offspring. It is frequently found that such stock 

 gradually "runs down" if continued indefinitely, as in the 

 case of Irish potatoes. 



2. Union of Similar Gametes from the Same Parent. 

 In cases where one kind of gamete is formed and the union 

 takes place between cells that have the same parent, we 

 have two nuclei taking part in the union, but they, having 

 the same ancestry, are likely to be very similar. Never- 

 theless it appears that such a union as this tends to 

 restore and strengthen the vitality of the embryo, as 

 compared with organisms in which no union occurs. This 

 is known as self-fertilization. 



3. Union of Unlike Gametes from the Same Parent. We 

 have seen that there are a good many hermaphrodite 

 parents, in which one parent produces both eggs and 

 sperm. The gametes are different, but the parent is one. 

 This is found, for example, in tapeworms, in earth worms, 

 in snails, in many plants. Doubtless self-fertilization 

 occurs in many of these cases. It is similar to the self- 

 fertilization described in the preceding section, except that 

 the gametes are distinctly different, are" formed in different 

 parts of the body, and their union introduces a little more 

 possibility of newness and variety in the result. 



4. Union of Dissimilar Gametes from Different Parents. 

 While self-fertilization is possible in the case of some of 

 the animals and plants which produce both kinds of 

 gametes in the same parent, it is not the rule. In a form 

 such as the earthworm or snail there are interesting 

 adjustments that tend to prevent the sperm of one animal 

 reaching and fertilizing the eggs of the same animal. In 

 both of these cases, when copulation between two animals 

 takes place, each animal transfers sperms to the other, and 

 later the sperms are brought into union with eggs of other 

 parentage. This is cross-fertilization. But the most 

 common and sure kind of cross-fertilization in the animal 

 kingdom is in the mating of parents which produce only 

 one kind of gamete, exclusively male parents mating with 

 exclusively female parents. In such case cross-fertilization 

 is insured by making self-fertilization impossible. 

 Separate maleness and femaleness is a device which 

 insures cross-fertilization. 



In the common seed-plants we have something quite 

 similar. Most of the plants produce both kinds of spores 

 in the same flower, and if the sperms produced by the 

 .pollen of a flower fertilize the eggs produced by the 

 gametophytes in the ovary of the same flower, we might 

 call it self-fertilization. We call it self-pollination, when 

 the pollen of one flower acts on the pistil of the same 

 flower. Self-pollination, however, seems to be the 

 exception in higher plants rather than the rule. Just as 

 in animals, there are many devices which prevent self- 

 fertilization, and secure cross-fertilization. In plants 

 there is a most wonderful series of adaptations tending to 

 insure cross-pollination and to discourage self-pollination. 

 Sometimes, as in the gametes in animals which have 

 separate sexes, a flower produces only pollen or only 

 ovules. In other cases, as in the hermaphrodite animals, 

 the two kinds of gametes in an individual flower do not 

 ripen at the same time. All of these facts show that nature 

 appears to favor cross-fertilization over self-fertilization. 



5. Evidence from Experiment. Charles Darwin was the 

 first to prove by actual experiment that cross-fertilization 

 has, at least in some forms, advantages over self- 

 fertilization. He took plants of one species and subjected 

 them to the same general conditions. Some he forced to 



