204 EVOLUTION OF TO-DAY. 



hybrids are not universally sterile, although they 

 usually are so in animals, but the presence of a few 

 exceptions is sufficient to show that there is no 

 rigid law separating species. We have seen that 

 the study of the animal kingdom reveals the fact of 

 some fundamental bond of union uniting the whole 

 together. This unity is shown by the possibility of 

 a classification which is not artificial but natural, 

 since actual relations between species are manifested 

 in the organic world. This unitary principle is shown 

 by the past history of the world as revealed by 

 fossils. There has been from the earliest times a 

 progression always manifesting itself. All the fossils 

 of the earlier times are such as belong to the same 

 types now existing, and many of them fall into 

 gaps between existing species, making the whole 

 history such as would have resulted from gradual 

 growth. It is shown most forcibly of all by the 

 fact that each individual of to-day in developing 

 from the egg repeats more or less exactly the past 

 history of its race as taught by fossils and such 

 other sources of evidence as are at hand. We have 

 seen that for this universal bond of union only two 

 explanations have been offered. The first places it 

 in the mind of the Creator, and leads to the theory 

 of types. This theory when examined has been 

 found so untenable in itself as to lead to its entire 

 abandonment. The second explanation discovers 

 the bond of union in heredity. This theory has 

 been found to raise objections of its own, some of 

 which are of no little importance. Most of these 

 difficulties, however, are plainly due to a lack of 



