THE PROBLEM 1 1 



of the apical or anterior region. In fact it is impossible 

 to escape the conclusion that certain general features 

 common to most or all axiate individuals are similarly 

 related to the polar apico-basal or antero-posterior axis, 

 as it is variously called. As regards other axes, the 

 differences in relation between them and the differences 

 in behavior and structure in different individuals 

 complicate the matter, but I shall show that there are 

 good grounds for believing that an organic or physio- 

 logical axis is fundamentally the same in all cases, 

 whether it is an axis of polarity or symmetry of a whole 

 organism or of a part. These geometrical relations 

 serve primarily to express in a general way the fact that 

 spatial order of certain kinds exists in the organic indi- 

 vidual, but the orderly sequence of events in time is also 

 referable to a greater or less degree to the geometrical 

 scheme of the individual or part. In development the 

 specialization and differentiation make their appearance 

 and undergo their progressive changes in a more or less 

 definite sequence with respect to the chief. axes. In 

 many cases the original geometrical plan of the indi- 

 vidual undergoes modification or gives place to a differ- 

 ent plan. Such changes are sometimes brought about 

 by conditions within the individual and sometimes 

 occur in response to changes in external conditions. In 

 general, however, it may be said that in any given kind 

 of individual the plan is always the same or undergoes the 

 same changes and is always worked out in essentially 

 the same way during development, provided external 

 conditions are the same. Under altered external con- 

 ditions departures from the plan may occur, and indi- 

 viduals result which differ more or less widely from the 



