296 EVOLUTION AND MANKIND, 



magnitude, accomplished by discontinuous leaps and bounds. De 

 Vries' own statement was that 



the new species appears all at once ; it originates from the parent species 

 without any visible preparation, and without any obvious series of transi- 

 tional forms. 



Further, these species are constant irom the first. The discon- 

 tinuous variations or mutations of de Vries are, in the main, the 

 outward manifestations of the presence or absence of the cor- 

 responding Mendelian factors. 



A Hormone hy{>othesis of heredity has l)een set forth by 

 Cunningham and others. A hormone is an internal secretion or 

 chemical messenger produced by some organ or tissue, such as 

 the reproductive organs, the pancreas, the thyroid gland and 

 actively growing tissues. Hormones " are produced in one organ, 

 and carried by the blood to another organ, on which their effect 

 is manifested." According to Cunningham, external stimulation 

 may affect certain hormones, and they, in turn, may influence the 

 gonads, producing modifications of the corresiK)nding parts of 

 the offspring. Support is said to be given to the hormone hyjx)- 

 thesis by observations on secondary sexual characters in animals, 

 though Geoffrey Smith interpreted such characters otherwise. The 

 development of antlers in the male deer would be the result of a 

 chemical stimulus originating in the male organs, which, by acting 

 on the skull, causes the inherited tendency to the development of 

 antlers to become active. An example of the disturbance of 

 hormones is afforded by the cretin, whose thyroid gland is dis- 

 eased, and to whom the administration of thyroid extract, 

 containing thyroid hormones to supply the deficiency, causes an 

 increase in mental and physical development. 



Charles Darwin ( 1809-1882) will always be reniembert-d 

 for the hypothesis of Natural Selection, published in 1859. It^ 

 was during his voyage on the " Beagle," some twenty-five years 

 before, that the evolutionary view of Nature began to unfold 

 itself before his mind. He found on the (valapagos Islands that 

 each island had its own distinctive animal i>opulation, especially 

 of reptiles and birds. He also noticed the striking resemblance 

 between the living and extinct animals of certain parts of South 

 America. He also made a large imniber of observations and 

 experiments on the breeding of animals under domesticated con- 

 ditions. He attached much importance to two factors, namely. 

 the existence of small, fluctuating variations, and the occurrence- 

 O'f a struggle for existence between organisms owing to the 

 natural increase in numbers and the resulting possible lessening 

 of the food supply. The possessors of the fitter variations tended 

 to survive. If this was kept up consistently, then by new adapta- 

 tions, and probably with the help of some form of isolation, new 

 species arose. Natural Selection was the process suggested l)y 

 Darwin to account for the origin of new species, by the preserva- 

 tion or survival of the fittest variations. The strength of the 

 hypothesis of Natural Selection lies in its interpretative value. 

 Darwin's hypothesis was supported by Alfred Russel Wallrtce 



