248 



SCIENGE. 



FN. S. Vol. VIII. No. 191. 



and of air, absence of exercise, movement, 

 etc. Also should be added horticultural 

 experiments which have been practised for 

 many years, as well as the results of accli- 

 miatization. 



Here should be mentioned the experi- 

 ments bearing on the mechanics of de- 

 velopment (Entwickelungsmechanik der 

 Organismen), or experimental embrj'ology, 

 of Oscar Hertwig, Roux, Driesch, Morgan 

 and others, and the curious results of ani- 

 mal grafting and of mutilations of the em- 

 bryos, obtained by Born and others, as well 

 as the regeneration of parts. The remark- 

 able facts of adaptation to new and un- 

 favorable conditions of certain embryos are 

 as yet unexplained, and have led to con- 

 siderable discussion and research. 



8. The a imori speculations of Darwin, 

 Galton, Spencer, Jaeger, N'usbaum, Weis- 

 mann and others, based on the results of 

 the labors of morphologists and cytologists, 

 have laid the foundation for a theory of the 

 physical basis of heredity, and for the sup- 

 position that the chromatine in the nucleus 

 of reproductive cells is the bearer of heredity. 

 The theory has already led to prolonged dis- 

 cussions aad opened up new lines of work 

 in cytology and embrj'ology. 



9. The subject of instinct, discussed from 

 an evolutional point of view, both by mor- 

 phologists and psychologists, particularly 

 by Lloyd Morgan, has come to the front, 

 while mental evolution has been discussed 

 by Eomanes and others. 



With all these theories before us, these 

 currents and counter- currents in evolu- 

 tional thought bearing us rapidly along, at 

 times perhaps carrying us somewhat out of 

 our depth, the conclusion of the whole mat- 

 ter is that in the present state of zoology it 

 will be wise to suspend our judgment on 

 many theoretical matters, to wait for more 

 light and to confine our attention mean- 

 while to the observation and registration of 

 facts, to careful experiments and to re- 



peated tests of mere theoretical assump- 

 tions. 



Meanwhile we may congratulate our- 

 selves that we have been born and per- 

 mitted to labor in this nineteenth century, 

 the century which in zoological science has 

 given us the best years of Lamarck's life, 

 a Cuvier, a Darwin, a Von Baer, an Owen, 

 an Agassiz, a Haeckel, a Spencer, and a 

 Huxley — the founders of modern zoology — 

 who have sketched out the grander features 

 of our science so completely that it will, 

 perhaps, be the work of many coming years 

 to fill in the details. 



GEOLOGICAL CAUSES OF VARIATION AND OF 



THE EXTINCTION AND RENEWAL OF 



SPECIES. 



The most immediate and efficient cause 

 of variation appears to be changes of en- 

 vironment or of the physical conditions of 

 existence. These, besides the agencies of 

 gravity, electricity, of the atmosphere, 

 light, heat, cold, food, etc., comprise geo- 

 logical changes or revolutions in the topog- 

 raphy of the earth's surface at different 

 periods. The latter causes appear to have 

 had much to do with the process of ex- 

 tinction and renewal of plants and ani- 

 mals. 



While the doctrine of the effect on ani- 

 mals of a change of environment was sug- 

 gested very early in this century and forms 

 the corner stone of Lamarckism, Wallace 

 was, after De la Beche,* and especially 

 Lyellf, the iirst in recent times, in an essay 

 published in 1855, to call attention to this 

 subject thus: 



" To discover," he says, " how the extinct 

 species have from time to time been re- 

 placed by new ones down to the very latest 

 geological period, is the most difficult, and 



*Eesearches iu Theoretical Geology. New York, 

 1837, p. 217. Quoted by Woodworth, p. 220. 

 t Principles of Geology, 1830-1832. 



