1901.] NATURAL SCIENCES OF PHILADELPHIA. 303 



THE LIMITS OF VARIATION IN PLANTS. 

 BY JOHN W. HAPvSHBERGER, PH.D. 



One of the most important questions on which the work of the 

 biologist should be brought to bear is the problem of species. We 

 see all living nature — animals and plants — divided into groups 

 which are denominated species. These groups are often clearly 

 and sharply defined, and, on the other hand, often very 

 irregularly characterized. What are the causes which have 

 brought this about ? What are the facts underlying the phe- 

 nomenon of species ? Two difficulties are presented to the earnest 

 student who attempts to formulate an answer to the above-men- 

 tioned questions. The well-known reasoning starts from the fact 

 that more animals or plants are born than can survive; some must 

 therefore perish and leave no descendants, and only those persist 

 which have structures and aptitudes that fit those organisms pos- 

 sessing them to bear their part in the struggle for existence. On 

 the whole, we find that the fittest will survive and breed. 



The first difficulty which presents itself is one which hangs on 

 the magnitude of the variations by which new forms ai*ise. What 

 are the limits of variation ? The older books on evolution consider 

 that the variations by which new species arise are at first small. 

 But if they are small, how can they be sufficiently useful to give 

 to those organisms possessing them an advantage in the struggle for 

 existence ? This is the difficulty of small or initial variations. 



The second difficulty is one known as that of the swamping efiiect 

 of intercrossing. Granting that variations do occur, how can they 

 be perpetuated ? For if the varying individuals breed with each 

 other, will not these variations be obliterated ? 



The following statistical study was undertaken with the purpose 

 of answering the first question, viz. : By what steps — by what 

 integral changes, of what size — did the new form come into exist- 

 ence? At the International Botanical Congress, held in Paris in 

 1900, M. Angel Gallardo spoke highly of the employment of the 



