Recent Developments in Heredity and Evolution 17 



material, biometry reveals the prevailing tendency, in 

 reference to these variations, in a group of individuals 

 representing a species, a tendency that could not be recog- 

 nized by the study of a single individual. Applying this 

 method to successive generations from this group of indi- 

 \-iduals, under experimental control, it can be disco\'ered 

 whether the prevailing tendency in the expression of these 

 variations remains the same, continuing the species as 

 before; or shifts, modifying the species as a whole; or 

 splits up, gi\'ing rise to a second marked tendency, that 

 ma}' mean presently two distinct species. 



It is evident that such data can be very suggestive, but 

 that their limitations must be recognized. They are data 

 concerning successive populations, and show the average 

 result of individual variation as expressed in a population. 

 In other words, they present in concrete and somewhat 

 definite form the problems of variation and inheritance 

 that must be solved. 



5. HEREDITY 



It must have become evident, during the preceding 

 sketch of representative theories of evolution, that the 

 fundamental factor in the process is variation, and that the 

 essential and inevitable question behind all of these explana- 

 tions is the origin of variation. This brings us at once to 

 the problem of heredity, with its supposed processes for 

 transmitting what we call "characters" from parent to 

 offspring. How is variation secured in this transmission ? 

 The earlier observers simply accepted variation as a fact, 

 and made no serious attempt to explain it. 



The first attack upon this ]:>roblem was the accumulation 

 of data in reference to the facts of heredity. To accumulate 



