352 



BIOLOGY AXD ITS MAKERS 



begins. Thus their history for thousands of years bears 

 testimony to the faet that the species have not remained 

 constant, but have changed into other species. 



Fig. 103 will give an idea of the varieties and gradations. 

 It represents shells of a genus, Paludina, which is still abun- 

 dant in most of the fresh waters of our globe. 



Fig. 103. — Transmutations of Paludina. (After Neumayer.) 



A similar series of shells has been brought to light in 

 Wiirttemberg in which the variations pass through wider 

 limits, so that not only different species may be observed, 

 but different genera connected by almost insensible grada- 

 tions. These transformations are found in a little flattened 



