THE PROBLEM OF ORGANIC DEVELOPMENT. 191 



processes underlying these steps must therefore be moving onward during the inter- 

 vals as well as during the steps. 



In such a series of stages the direction of progress is certainly as definite as 

 the bird is true to its type. The steps are seriated in a causal, genetic order an 

 order that admits of no transpositions, no reversals, no mutation-skips, no unpre- 

 dictable chance intrusions. The series may conceivably be lengthened or shortened , 

 strengthened or weakened; indeed, we may multiply the number of steps at will; 



FIG. 34. Geopelia cuneata, age 68 days. Bird in the second plumage, which is like that of adult. 



that is, we may provoke one or more steps to arise between any two normal steps; 

 but in that case the new steps will be measured true to the time and place of intro- 

 duction, and their direction will invariably coincide with that of the series as a 

 whole, so that if the time and place of origin are noted, the nature and extent of 

 the strides may be approximately predicted. 



Specific characters often appear to come in suddenly and to be immutable. This 

 seems often to be so in passing from the Juvenal to the mature plumage, with one or 

 several molts. The earlier color-pattern is often succeeded by one so different that 

 we are puzzled to see any possible way of continuous differentiation from one to the 



