12 ELEMENTS OF PALAEONTOLOGY 



and perfection existing in each individual. According to Lamarck, new 

 properties brought about by these influences are transmitted to descendants 

 through inheritance, and become permanently established in the race. Geoffroy 

 St. Hilaire maintained the same principles on the whole, but ascribed the chief 

 causes of variation of species to the influence of environment. 



The Darwinian theory of natural selection is based upon the property 

 common to all organisms of acquiring ancestral characteristics through heredity, 

 and of transmitting them in turn to their progeny ; and also on the adapta- 

 bility of organisms to particular external conditions, by means of which 

 variations are brought about. Since in the struggle for existence only those 

 individuals which are the best adapted that is to .say, those possessing the 

 most advantageous modifications survive, nature is continually exercising, 

 according to Darwin, a most rigorous selection which operates toward the 

 increase and perfection of useful variations. Through the constant accumula- 

 tion of originally slight yet serviceable modifications, and through the perpetual 

 transmission of the same from one generation to another, there are produced 

 first different varieties, then species, and eventually genera, families, and orders. 

 The zoological and botanical classifications are, according to Darwin, merely an 

 expression of genealogical facts, exhibiting the remoter and closer ties of con- 

 sanguinity which exist among different organic forms. 



Darwin's explanation of the origin of species through the agency of natural 

 selection found in Wallace, Huxley, Haeckel, and others, zealous and ingenious 

 supporters, while on other sides it encountered vehement opposition. M. 

 Wagner regarded free intercrossing as an insurmountable obstacle to the 

 establishment of new modifications, and contended that the isolation of a few 

 individuals, a condition which would occur most frequently during migrations, 

 was a necessary postulate in accounting for the origin of each new variety or 

 species. Bronn, Nageli, and A. Braun raised the objection to Darwin's theory 

 of natural selection that many organs are entirely useless to the individual, 

 and therefore natural selection, which depends upon the principle of utility, 

 could neither have produced such organs nor could have modified them in any 

 way. Nageli assumed that, in addition to natural selection, a certain resident 

 tendency toward perfection, inherent in every individual, takes part in con- 

 ditioning the growth of morphological characters. Every variation brought 

 about by external or internal agencies is at once in the nature of a differentia- 

 tion, a step forward in the division of labour, and consequently an advancement. 



Weismann endeavoured in a similar manner to supplement Darwin's theory 

 of selection by his hypothesis of the continuity of germ-plasm. According to 

 Weismann, germ-matter is of itself capable of producing all variations that 

 are useful to an organism. Only that which exists in the original plasm or in 

 the sexual elements as embryonic rudiments can be transmitted to offspring 

 and become further acted upon and developed by natural selection, according 

 to Weismann's theory. The continuity, that is to say, the perpetual trans- 

 mission of a portion of the germ-plasm from parents to offspring, forms a 

 necessary postulate to the theory of descent. In opposition to Weismann, 

 who attributes only a subordinate influence to the action of physical environ- 

 ment as a cause of variations, and who particularly denies the inheritance of 

 acquired characters, stands the Neo-Lamarckian school (represented by Herbert 

 Spencer, Cope, Hyatt, Osborn, Semper, Glaus, Roux, and others), which ranges 

 itself more and more on the side of Lamarckian ideas, and ascribes to the use 



