Ihave collected so large a body of with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a general law of nature that no organic being fertilizes itself for a perpetuity of generations; but that a cross with another individual is occasionally—-perhaps at long intervals of time—indispensable. in this sentence from the third paragraph of the passage, what does perpetuity mean? a) gratefulness b) consideration c) a temporary state d) time without end; eternity