12 THE PRINCIPLES OF SCIENCE. 



fraction of those which are in the neighbourhood of our 

 system, we must accept Kepler's opinion, that there are 

 more comets in the regions of space than fishes in the 

 depths of the ocean. When like calculations are made 

 concerning the numbers of meteors visible to us, it is 

 astonishing to find that the number of meteors entering 

 the earth's atmosphere in every twenty-four hours is 

 probably not less than 400,000,000, of which 13,000 

 exist in every portion of space equal to that filled by 

 the earth's globe. 



Most serious fallacies may arise from overlooking the 

 inevitable conditions under which the records of past 

 events are brought to our notice. Thus it is only the 

 durable objects manufactured by former races of men, 

 such as flint implements, which can have come to our 

 notice as a general rule. The comparative abundance of 

 iron and bronze articles used by an ancient nation must 

 not be supposed to be coincident with their comparative 

 abundance in our museums, because bronze is far the 

 more durable. There is always a prevailing fallacy that 

 our ancestors built more strongly than we do, arising from 

 the fact that the more fragile structures have long since 

 crumbled away. It is thus that we have few or no relics 

 of the habitations of the poorer classes among the Greeks 

 or Romans, or in fact of any past race ; for the temples, 

 tombs, public buildings and mansions of the wealthier 

 classes alone endure. There is an indefinite expanse of 

 past events necessarily lost to us for ever, and we must 

 generally look upon records or relics as exceptional in 

 their character. 



Exactly the same considerations apply to geological 

 relics. We could not generally expect that animals would 

 be preserved, unless as regards the bones, shells, strong 

 integuments, or other hard and durable parts. All the 

 infusoria and animals devoid of mineral framework must 



