LECTURE XXXV 

 THE ORIGIN AND THE EXTINCTION OF SPECIES 



Adaptation does not dej^end upon chance — The case of eyes — Of leaf-mimiciy — 

 All persistent change depends ultimately on selection — Mutual sterility without great 

 significance— Eelative isolation {Lepus variabilis)— InHuence of hybridization — Deca- 

 dence of species— Differences in the duration of decadence — Natural death of individuals 

 — Extinction due to excessive variability (Emery) ?—Machairodits as interpreted by 

 Brandos— Lower types more capable of adaptation than higher — Flightless birds — 

 Disturbance of insular fauna and flora by cultivation — The big game of Central Europe. 



In the polar hare we have a case in which the adaptations to the 

 life conditions both of time and space are recognizable as the effect 

 of definite causes, and thus as a necessity ; but the same must be true 

 everywhere even in regard to the most complex adaptations which 

 seem to depend entirely upon chance ; everywhere adaptation results 

 of necessity — if it is possible at all with the given organization of the 

 species — as certainly as the adaptation dress of the hare depends 

 on the length of the winter, and in point of fact not less certainly 

 than the blue colour of starch on the addition of iodine. The most 

 delicate adaptations of the vertebrate eye to the task set for it by 

 life in various groups have been graduallj^ brought about as the 

 necessary results of definite causes, just in the same way as the 

 complex protective markings and colouring on the wing of the 

 Kallima and other leaf-mimicking butterflies. 



That adaptations can be regarded as mechanically necessitated 

 is due to the fact that in every process of adaptation the same 

 direction of variation on the part of the determinants concerned is 

 guaranteed, since personal selection eliminates those which vary in 

 a wrong direction, so that only those varying in a suitable direction 

 survive, and they then continue to ya,ry in the same direction. But 

 the greatest difference between our conception of natural selection 

 and that of Darwin lies in this : that Darwin regarded its intervention 

 -.as dependent upon chance, while we consider it as necessary and 

 conditioned by the upward and downward intra-germinal fluctuation 

 of the determinants. Appropriate variational tendencies not only 

 may present themselves, they must do so, if the germ-plasm contains 

 determinants at all by whose fluctuations in a plus or minus direction 

 the appropriate variation is attainable. 



