352 BIOLOGY AND ITS MAKERS 
begins. Thus their history for thousands of years bears 
testimony to the fact 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. 
4 
Fic. 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 
