46 EVOLUTION OF TO-DAY. 



by minute gradations. These connecting links are 

 not so prominent elsewhere, but they can be found 

 in all groups of animals. It is not to be understood 

 from this, however, that there is no difference be- 

 tween species and varieties ; the former show much 

 greater differences than the latter, and are usually 

 more stable. But there is no marked line between 

 them. If we examine the relations of species to each 

 other and compare them with the relations of varie- 

 ties, a strict parallel can be drawn. The large num- 

 ber of domestic pigeons, with their numerous races, 

 can be divided into groups and sub-groups corre- 

 sponding exactly to the divisions which are found 

 in nature of genera and species. It is only neces- 

 sary to imagine the mutual fertility to disappear, 

 and these groups would resemble in every respect 

 the natural grouping of genera and species. In 

 other words, the working of nature in accumulating 

 variations produces results which, if carried far 

 enough, would give relations exactly such as are 

 found existing between species. 



Turning to fossil deposits there is in some cases 

 evidence amounting to practical proof that species 

 can arise in this way. The best illustration of this 

 is found in the Steinheim Lake deposit. At Stein- 

 heim, in Wurtemberg, there is a large deposit of 

 shells which has been subject to special study. The 

 deposit represents the bed of an old lake, and it 

 appears that in Tertiary times there migrated into 

 this lake a species of fresh water mollusk, called 

 Planorbis. This animal found an unoccupied field 

 in this lake, and multiplied rapidly. As fast as the 



