DIFFICULTIES TO BE OVERCOME. 145 



what to look for, and in a subject as difficult for ob- 

 servation as this, it is almost necessary to know what 

 to expect. In short, all of the errors belonging to 

 the infancy of a subject are found in the older in- 

 vestigations, and it has been necessary, therefore, to 

 make numerous corrections. They are constantly 

 being made, and with a new understanding of what 

 is significant and what unimportant, new interpre- 

 tations are being gained. From the general uncer- 

 tainty in regard to former work, it is necessary now 

 to review almost everything which was done earlier 

 than twenty-five years ago, if one wishes to make 

 sure of any important point. The modern student 

 has found that, so far as superficial observations 

 were concerned, the older embryologists were accu- 

 rate and painstaking in what they saw, but the 

 internal anatomy, and particularly the changes 

 which take place in various stages, they were likely 

 to describe incorrectly. 



Further difficulty has arisen out of the fact, al- 

 ready dealt with at some length, that the embryo- 

 logical history is so frequently modified. Not only 

 has food-yolk produced a series of modifications, 

 but another entirely different sort of changes has 

 arisen in free larvae. Many animals pass part of 

 their early development as free-living, independent 

 organisms, as independent, indeed, as the adults. 

 The caterpillar is nothing more than an embryonic 

 stage of the butterfly, and is sometimes more inde- 

 pendent than the butterfly itself. Such larvae, living 

 as free forms, obtain their food by their own exer- 

 tions, and have many enemies to contend with. 



