208 Geological Evidence of Origin of Species. 



founding the two ? This may be true, yet every naturalist 

 knows there are numerous cases in which the limits of so- 

 called species are unfixed and unfixable. "What one regards 

 as a species another maintains is a variety. Then indi- 

 viduals intermediate in character between the species and 

 the variety are discovered, and the variety is abolished. 

 New discoveries fill in the gap between two species, and 

 one of the species is abolished. The common Devonian 

 fossil Spirifera disjuncta was once eighteen species, but the 

 different forms so graduated into each other that it was 

 found impossible to maintain the old distinction. In this 

 state, there is a fossil which ranges from the Clinton group 

 up to the Chemung, L e., from the lower Silurian (Murchi- 

 son's base of upper Silurian) to the upper Devonian. In 

 each formation it puts on certain characters (not always 

 dependent on the nature of the sediment in which it lived) 

 which will enable the experienced geologist to decide the 

 formation by the fossil. This species is described by Prof. 

 Hall as cetrypa reticularis. Some palaeontologists would 

 make half a dozen species out of it. This is the class ©f 

 difficulties we meet with in defining species, difficulties 

 which are themselves arguments in favor of evolution, and 

 which, indeed, will lead me to my first proposition. 



1. Fossils fill in many of the gaps which formerly separated living 



types. 



Let us first enquire the value of such evidence. If, be- 

 tween two living forms, A and C, a fossil combining many 

 of the characters of both be discovered, does it prove that 

 C was evolved from A or A from C, through the interme- 

 diate stage B ? Not necessarily. The discovery of B sim- 

 ply supplies us with one of the stages by which evolution 

 may have taken place, L e., it removes some of the improba- 

 bility that two very distinct forms should be connected by 

 actual descent. Take an illustration. John Plantagenet, 

 of Vermont, claims descent from Richard III of England. 



