ON TIIK NATl'KK AM) BASIS OF II KHKD1TV. 187 



energy? I think the male in many species of birds passes directly through and 

 beyond the female stage. In many cases even the females may now and then pass 

 beyond the normal female and advance towards the male condition. The energy of 

 development and degree of fertility appear to be correlated. (W 6) 



If fertility is a thing of all degrees, and that too within the limits of a single 

 species, are not the qualitative distinctions inferred therefrom likewise things of 

 degrees, and variable within the same limits? If fertility may vary individually, as 

 is certain, is there any reason to doubt that its physical bases are equally variable 

 from individual to individual? But fertility varies also according to age, conditions 

 of food, temperature, etc. How then can qualitative differences be an infallible 

 criterion of species? 



We have weakness, not only in ova, but also in the spermatozoa of hybrids. 

 Guyer 19 showed that the sperm of hybrids (from wide crosses) do not come to full 

 development. The hybrids on which he worked were hybrids between common 

 pigeons and ring-doves, and these hybrids are practically all males. Among other 

 hybrids from very wide crosses (as the common dove and Japanese turtle) I have 

 had one fertile male. In hybrids between ring-doves and Japanese turtles I have 

 had quite a number of fertile males and some females. As hybrids are taken between 

 more and more nearly related species the sperm rises to more and more perfect 

 development. As development is more complete, energy is more complete and 

 fertility becomes highest. 



Early fertility is of ten followed by later infertility. For two years (1907 and 1908) 

 I have mated a male Japanese turtle with a female hybrid between the Japanese 

 turtle and a ring-dove. Each time fertility has been equal to development and 

 hatching in the height of the season; then perhaps a few eggs were fertilized, but 

 this gradually fell short, until no sign of beginning development could be seen. 

 This means that the season of greatest vigor of the birds is the season of highest fertility. 



Infertility may often be found associated with "weakness." Birds in the first 

 breeding season do not do as well as when they are 3 or 4 years old; only then do 

 they come to fullest maturity, and they then have all their surplus energy for 

 reproduction. 



Just as energy gives fertility, so it carries development to different heights, e.g., 

 males above females. 20 Birds of resplendent plumage or increased number of feathers 

 (fantail, Japanese cock, peacock, etc.), are birds that have attained greater and 

 greater developmental energy. In the same way those tissues of greatest energy 

 carry the number of parts, and differentiation, to the highest points. 



Problems of evolution are to be slowly approached through observation, expe- 

 riment, reflection, and theory. Their solution will tax the resources of the labora- 

 tories in every department of biology, indeed of every department of science; and 



1J This author hus since called attention to the fact that in still some other orders of birds a majority of hybrids 

 are males. EDITOR. 



20 "The coexistence of the sexes is known in a number of instances. Nansen showed that Myxinc glulinosa is a 

 male until 32 cm. long, and thereafter produces ova. In some cases only male cells are produced. This does not 

 look like sex-determination by a special chromosome. Schultze, working with Microstoma, showed that in rare cases, 

 when fission begins, the anterior zooid is male, the posterior female. Ishikawa (1891) showed that in the shrimp 

 Gebia the gonad has anterior tosticular and posterior ovarial parts." (XZ 4) 



