2/4 THE FOUNDATIONS OF ZOOLOGY 



and the people of the most enlightened nations eighteen hundred 

 years ago ; and it remained the accepted doctrine of learned and 

 unlearned Europe through the Middle Ages, and even to the 

 seventeenth century." 



It is clear that natural selection would have given the death- 

 blow to the argument from contrivance if this opinion had been 

 well founded; but it is equally well known that the progress of 

 science has shown the worthlessness of all the evidence for spon- 

 taneous generation. 



In my opinion the second alternative is most consistent with 

 the present state of our knowledge; for while the discovery of 

 natural selection has shown how all the endless forms of life, with 

 all their admirable and wonderful adjustments to the diversity and 

 harmony of the external world, may have arisen from a common 

 starting-point in some primitive organism, so simple and so homo- 

 geneous that its production out of inorganic matter does not seem 

 improbable, the progress of our knowledge in other lines has 

 demonstrated that, as a matter of fact, all the living things we 

 know do arise from preexisting living things. 



The demonstration of the continuity of life which we owe to 

 the embryologists and histologists of modern times, and to the 

 students of pathology and hygiene, is a contribution to philosophy 

 of the utmost value and significance. This law of continuity is a 

 discovery as real as the law of natural selection itself, for we now 

 have every reason to believe, not only that personal identity is 

 coextensive with life, but also that there is no break in its conti- 

 nuity at any point in the whole history of life. Every living thing 

 on earth, and, so far as we know, all that have ever lived, are 

 personally identical with the primeval living being, in exactly 

 the same sense as the mature, conscious, rational man is person- 

 ally identical with the human foetus and the new-born babe. 



The history of the great modern discovery of the continuity 

 of life has been written by so many able students that there would 

 be no reason to review any part of it here if the share of that 

 great investigator and thinker, William Harvey, in the demonstra- 

 tion that the facts are, in this matter, opposed to venerable author- 

 ity, had not been so strangely misunderstood and misrepresented 

 as to call for correction. 



