xii CONTENTS. 



PAGE: 



mutations theory, 368. References to theories explaining 

 secondary sexual characters, 373. 



CHAPTER XII. 



DARWINISM'S PRESENT STANDING ... 374 . 



Natural selection the final control in evolution, but not 

 a sufficient explanation of species-forming, 374. Weak- 

 nesses of the substitutionary theories, 375. The unknown 

 factors of evolution, 377. Prime needs of evolution study; 

 first, the intensive study, statistical and experimental, of 

 variability, 378. Second, the means of cumulating varia- 

 bility along definite lines, 379. Third, the investigation of 

 adaptation, 380. A suggested argument for a belief in the 

 transference of ontogenic changes into phylogeny, 382. A 

 suggested automatic causal factor of variability, purposive 

 but not purposeful, 384. Our present ignorance and the 

 call to work, 387. 



APPENDIX: Wigand's criticism of the selection theories, 

 387. Lankester's upholding of Darwinism, 389. De Vries's 

 discussion of species-forming by selection, 389. Delage's 

 estimate of selection, 390. Osborn's championship of the 

 unknown factors of evolution, 391. Klebs's conclusions 

 from experiments on plants, 392. Friedlander's discussion 

 of adaptation, 392. Loeb's attitude toward the problem of 

 species-forming, 393. 



INDEX . 397 



