Continuity and Divergence 73 



maggots. The second of these principles is so readily observed 

 that only the simple-minded hope to change geese into swans. 

 Whether one is familiar with biology or not, he would be 

 astounded to see pumpkin seeds grow into pine trees, or a cow 

 give birth to kittens. 



Our very familiarity with the idea that like begets like 

 stands in the way of our recognizing the equally important 

 and equally established fact that offspring are never exactly 

 like their parents. It makes difficult the thought that new 

 kinds of plants and new kinds of animals do actually appear, 

 not only at rare intervals as " freaks," but regularly in every 

 garden, along every roadside, in the ocean, in the woods, on 

 every farm. We notice monstrosities, especially if they are 

 unable to maintain themselves, or to reproduce themselves. 

 We do not notice the more frequent appearance of new com- 

 binations of characters that are not very striking at first but 

 that may nevertheless give rise to a new strain or breed of 

 plants or animals. There is involved here a third set of facts. 

 There is resemblance between parents and offspring, like does 

 indeed beget like: but the resemblance among the individuals 

 having the same ancestors is never perfect. Nor is the re- 

 semblance between individuals and their ancestors perfect. 

 Parents resemble their offspring on the average more than 

 they do strangers, but less than do brothers and sisters. In 

 general, we measure the degree of relationship by the degree 

 of resemblance. 



This brings us to a fourth set of facts: offspring differ 

 from their parents and their ancestors. There is not only 

 descent from ancestors, but alsa divergence in form, color, 

 size and other qualities which are characteristic in the suc- 

 cessions from generation to generation. 



These facts together constitute the very essence of 

 the principle of organic evolution — namely descent with 

 modijication: 



(i) So far as we know, all life comes from previously 

 existing life; there is continuity of life, through reproduc- 

 tion, from generation to generation. 



