THE ORIGIN AND THE EXTINCTION OF SPECIES 353 



or eternal, but that they change in the course of the earth's history. 

 The numerous fossil remains in the various strata of the earth's crust 

 prove that this is true in a high degree, that in almost every one of 

 the more important geological strata new species occur, and that not 

 only species and genera, but families, orders, indeed whole classes of 

 animals, which lived at one time, have now completely disappeared 

 from the face of tlie earth. We can understand this phenomenon 

 when we reflect that the conditions of life have also been slowly 

 changing through the course of the earth's history, so that the 

 old species had only the alternative of dying out, or of becoming 

 transformed into new species. 



But simple as this conclusion is, it can hardly be deduced with 

 certainty from the occurrence and succession of the fossil species 

 alone. For instance, we should strive in vain to recoo-nize the cause 

 which led one of those regularly arranged snail-species of the 

 Steinheim lake basin to become transformed into one or two new 

 species at a particular time, or to And the cause which moved those 

 curious tripartite Crustaceans of primitive times, the Trilobites, which 

 peopled the Silurian seas with such a wealth of forms, to become 

 suddenly scarce towards the end of the Silurian period, and to dis- 

 appear altogether in the succeeding period, the Devonian. The 

 famous geologist Neumayr sought to refer this striking phenomenon 

 to the fact that just at that time the Cephalopods, "the most formid- 

 able and savage marauders among the invertebrate marine fauna,' 

 gained the ascendancy, and it is quite possible that he was right 

 in his surmise, but who is to prove it "? Can we decide even in the 

 case of animals now livino- whether the losses inflicted on a much 

 persecuted species by an abundant and greedy persecutor exceed the 

 numbers of progeny, and are therefore driving the species gradually 

 towards extermination ? Probable as such a supposition appears, it 

 cannot be accepted as proven. 



Since in many cases of the extinction of great animal-groups we 

 cannot even prove that there was a simultaneous ascendancy of power- 

 ful enemies, other factors must be discovered to which the apparently 

 sudden disappearance may be attributed. Many naturalists have 

 tried to guess at internal reasons for extinction, and have adopted the 

 theory — associated with the tendency to assume mystical principles of 

 ■evolution — that species in dying out are obeying an internal necessity, 

 as if their birth and death were predestined, as it is in the case of 

 multicellular individuals, as if there were a physiological death of the 

 species as there is of the multicellular individual. 



Neumayr showed, however, that the facts of palaeontology afford 

 II. A a 



