368 ELEMENTS OF BIOLOGY 



and night, for example, are constantly succeeding each other. The 

 organism is therefore constantly changing in accord with the en- 

 vironmental changes. These modifications imposed on the organ- 

 isms by its surroundings are superim^nosed on alterations in the 

 organism with age and other conditions. Not only is the individual 

 constantly changing; it is also changing in its ability to change. 

 Moreover, ability to adapt is not identical in all organisms. Ability 

 is itself a species character. So our attempt to group adaptations as 

 species adaptations and individual adaptations is subject to this 

 modification. 



Trembley's Experiment with Hydra. From a great wealth 

 of examples of individual functional adaptations, a few may be se- 

 lected to illustrate their nature and range. Even the most simple 

 forms are often able to solve difficult problems. One of the most 

 striking cases was first brought to notice over one hundred and 

 fifty years ago by the experiments of a French priest, Trembley, on 

 hydra. The objectives of his extended investigations of hydra do not 

 concern us here, but in the course of his experiments he attempted 

 to find if the ectoderm and endoderm are interchangeable, whether 

 if their positions were reversed, the characters of the two layers 

 would change, ectoderm become endoderm and vice versa. 



He first cut of? the tentacles and then by means of fine instru- 

 ments turned the animals inside out. When he returned later 

 to examine the animals, he found the ectoderm on the outside and 

 the endoderm on the inside. He suspected that they had righted 

 themselves, and subsequent observations showed that to be the case. 

 Next he repeated his operation of turning the hydra inside out, 

 but this time he impaled them on bristles, so that the animals were 

 suspended as sacks might be suspended on rods run through them. 

 On returning he again found the ectoderm outside and the endo- 

 derm inside. Being unable to determine how a sac-like structure 

 could have its inner and outer surfaces reversed while suspended in 

 this fashion, he concluded that a transformation of the two layers 



