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Show CHAP. XIV. RECAPITULATION. 460 ° t 1 difficult even to conJeC ure It is no doubt, extreme y t uctures have been per-by wh'a t gradati.o ns many s r st broken and f a1' l1' ng · lly among fected ' more es. pebcl ~lU <YS. b Ut we see so many strange aroups of organic e b ' • proclaimed by the canon, o . t e as IS b <Yradations 1n na ~r ' lt , that we ought to e exb" Natura non "' cit sa um, . t' t 1a . . that any organ or 1ns Inc , t . lU saying c: • t tremely cau lOUS ld t llave arrived at Its presen b . . cou no . b or any whole eing, d teps. There are, 1t must e state by many graduate . s h f 1 difficulty on the t eory o of speCia . f admitted, cases d f the most cunous o t 11 ese l t' . an one o natural se· ec 10n 'f two or thr ee defined castes of workers is the e. xistenc1e 0 ·n t h e same community of ants; but or stenle fema es 1 h how this difficulty can be I have attempted to s ow mastered. . h lmost universal sterility of With r spect to t e da h' ch forms so remarkable 1 first crosse , w 1 · t' specie w l n t ·versal fertility of vane Ies 'th the almo unl . l a contrast Wl £ the reader to the recapitu a· when crossed, I m~st r:::he end of the eighth cha~t~r, tion of the facts given l . 1 to show that this stenhty t me cone us1v Y . ·t which seem 0 . d ment than is the 1ncapam Y e 1al en ow .. · · is no mor a sp f d t ther . but that It IS mm· t b gra t og ' d t' of two tr . . l differences in the repro ue Ive d ntal on con tit uti na . vV e see the truth . t . ro sed peCl s. lt sy t m f th ~n er. the vast difference in t~e resu : of thi c nclu 1 n In . ·e crOEssed reciprocally' two Cies ar 1 d wh n . th } n l) C.l . fu·st used as the fat ler an that S IS 1 , w 1 n 11 . th n , th n1 th r. . . when intercrossed and ~f r h f rtility f v~rwtl s t be considered as .~m· tl . . ro ngr ·1 ffi ·prlng c. anno . l £ tility surprismg 1 u . y genera er · h .. 1 . n r i th ll' ver . . t likely that eit er Y I ' ' that Jt lS no h uld wh n w ~ Jn. n<b r th ir r productive systems s t the th ir n ·tituti n r d. fi d Moreover, roost o hav b n profounilly roo 1 e . CIIAP. XIV. RECAPITUI,ATION. 461 varieties which have been experimentised on have been produced under domestication; and as domestication apparently tends to eliminate sterility, we ought not to expect it also to produce sterility. The sterility of hybrids is a very different case from that of first crosses, for their reproductive organs are more or less · functionally impotent; whereas in first crosses the organs on both sides are in a perfect condition. As we continually see that organisms of all kinds are rendered in some degree sterile from their constitutions having been disturbed by slightly different and new conditions of life, we need not feel surprise at hybrids being in so~e degree sterile, ~or their constitutions can hardly fail to have been disturbed from being compounded of two distinct organisa .. tions. This parallelism is supported by another parallel, but directly opposite, class of facts ; namely, that the vigour and fertility of all organic beings are increased by slight changes in their conditions of life, and that the offspring of slightly modified forms or varieties acquire from being crossed increased vigour and fertility. So that, on the one hand, considerable changes in the conditions of life and crosses between greatly modified forms, lessen fertility ; and on the other hand, lesser changes in the conditions of life and crosses between less modified forn1s, increase fertility. Turning to geographical distribution, the difficulties encountered on the theory of descent with modification are grave enough. All the individuals of the same species, and all the species of the same genus, or even higher group, must have descended from common parents ; and therefore, in however distant and isolated parts of the world they are now found, they must in the course of successive generations have passed from some one part to the others. We are often wholly unable |