230 DEVELOPMENT AND HEREDITY. 



series of stimuli that have acted separately on the 

 two ancestral lines, then we might expect the indi- 

 vidual development to combine characteristics of the 

 two parents, — the mixture of stimuli calHng forth a 

 corresponding mixture of developmental reactions. 



Experiments in crossing breeds, varieties, . and 

 species of plants and animals, show that the heredi- 

 tary impulses of the two organisms united in the 

 cross must be very similar if a perfect development 

 is to result. When the two organisms differ enough 

 to be classed as separate species, the offspring of the 

 union is rarely perfect. Two allied species have 

 the greater part of their development ahke, — it is 

 only as their development nears its completion that 

 the lines diverge. In the same way, they have 

 had a common ancestor in comparatively recent 

 times, and before their divergence from that ances- 

 tor, they have had a common ancestral history. 

 From what we have learned of the origin of the 

 hereditary impulse, we may conclude, therefore, that 

 two allied species have their hereditary impulses 

 identical up to a certain stage of growth — the stage 

 represented in their racial history by the common 

 ancestor ; from this point, through the latter part of 

 their development, the hereditary impulses proceed 

 in opposite directions. The offspring of a cross of 

 two such species might therefore continue its devel- 



