12 ELEMENTS OF PALEONTOLOGY 



The Darwinian theory of natural selectioii 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 of all new 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 

 consanguinity which exist among difFerent organic forms. 



Darwin's explanation of the origin of species by the addition of the 

 agency of natural selection to the Lamarckian factors of Variation and 

 inheritance found in Wallace, Huxley, Haeckel and others, zealous and 

 ingenious supporters, although on other sides it encountered vehement 

 Opposition. Moritz 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 w^ould occur most frequently 

 during migrations, was a necessary postulate in accounting for the origin of 

 each new variety or species. As will be stated presently, the principle of 

 isolation, slightly modified, has been applied by other writers. Bronn, Nägeli 

 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. Nägeli 

 assumed that, in addition to natural selection, a certain resident tendency 

 toward perfection, inherent in every individual, takes part in conditioning 

 the growth of morphological characters. Every Variation brought about 

 by external or internal agencies is at once in the nature of a differentiation, 

 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 off'spring 

 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 parent to ofFspring, forms a 

 necessary postulate to the theory of descent. 



Weismann originally attributed only a subordinate influence to the 

 action of physical environment as a cause of variations, and particularly 

 denied the inheritance of acquired characters. But in his later writings, 

 he is inclined to admit that somatic variations due to environmental influences 

 may be transmitted to the ofFspring, and endeavours to explain this with the 

 help of his germ-plasm hypothesis. Thus he approaches in a way the opinion 

 of his opponents, the so-called Neo-Lamarckian school (represented by Herbert 



