304 THE EVOLUTION THEORY 



one had to deal with, and the more difficult the more numerous these 

 became, because with the number of individuals, especially if they 

 come from a wide area, the number and diversity of the divergences 

 increases also, so that in many cases, as in that of the Celebes snails, 

 it becomes impossible to draw a line between the different species. 



There are, however, many animal and plant forms which do not 

 show such marked divergences, but rather exhibit a great harmony 

 of individuals even in detail, and the conception of species is more 

 readily applied to these. It would certainly be foolish to give it up, 

 since we should then lose all possibility of arriving at any sort of 

 orientation among the enormous wealth of forms in nature. But at 

 the same time we must not forget that these ' typical ' species only 

 appear so to our short-sighted vision — short-sighted as far as time is 

 concerned — and that they are connected from long-past times with 

 ' species ' which lived at an earlier date, by just such transition stages 

 as connect the Celebes species of to-day, which are all living at the 

 same time. The world of life on the earth onl}^ presents at any given 

 time a ' cross- section of its genealogical tree,' and according as its 

 branches grow out vertically or horizontally we receive an impression 

 of typical, sharply defined species or of circles or chains of forms. 

 In the first case the evolution of new species was associated with the 

 dying out of the horizontal branches, and the end- twigs of the branch 

 stand beside each other now apparently isolated and sharply defined ; 

 in the other case only a portion of the ancestral species has been 

 transmuted, and the other part continues to live alongside of the 

 species derived from it, and perhaps repeats the process of giving 

 off a varied race of descendants. 



The last thirty years have yielded much palaeontological evidence 

 of the successive stages of species-transformation. In quietly de- 

 posited horizontal strata of the earth's crust, lying one above another, 

 the whole phyletic history of a group of snail-species has repeatedly 

 been found in historic order, the oldest in the deepest layer, the 

 3^oungest in the uppermost, and the numerous and often very divergent 

 ' species ' of a particular deposit are connected by transition forms 

 in the intermediate strata. From the point of view of time, therefore, 

 these are not ' typical ' species, but circles of forms in a state of 

 variability. 



The most beautiful of such cases are the Planorhis species from 

 the small lacustrine deposits of Steinheim in Swabia, the Paludina 

 strata of Slavonia, and various groups of Ammonites. 



These cases have been described and discussed so often that 

 I need only refer to their most essential features. 



