ON PHYSIOLOGICAL MORPHOLOGY. 4 1 



soon cover a large area of the glass. In this way the apical 

 end of a stem may continue to grow as a totally different 

 organ, namely, as a root. Every organ not in contact with 

 some solid body gives rise to polyps. Even the main root, if 

 not in contact with a solid body, no longer grows as a root, but 

 gives rise to a great number of small polyps which appear at 

 the end of long stems. Fig. 7, which Mr. Tower was kind 

 enough to draw for me, shows a branch which formed roots 

 at its apex and polyps at its roots in this way. The stem 

 touched the bottom of the dish with the apical ends, a, b, c and d. 

 All these ends gave rise to roots. From the upper side of the 

 original root, r, which was not in contact with the glass, later 

 on small polyps, //>, grew out. Every place which was in con- 

 tact with solid bodies gave rise to roots, and every place which 

 was in contact with sea water gave rise to polyps. 



This is not the only species of hydroid found at Woods Holl 

 in which such forms of Heteromorphosis can be produced. 

 Another form, Pennaria, is just as favorable. In Pennaria I 

 succeeded repeatedly in producing roots at both ends of a 

 small stem that bore no polyps. ^ 



What circumstances are common in these experiments on 

 Margelis and Pennaria.? Organs brought into contact with 

 solid bodies continue to grow as roots, if they grow at all. 

 Organs surrounded on all sides by water continue to grow in 

 the form of polyps, if they grow at all. In Margelis, contact 



1 In a Tubularian I was able to produce the opposite case, namely, to get an 

 animal that ended at botli ends in a polyp and had no root. Weismann seems 

 to assume, in his " Germ Plasm," that the latter result is to be explained by the 

 principle of natural selection, inasmuch as an animal without polyps could not 

 continue to live, and hence it would be impossible to produce roots at both ends. 

 In Pennaria this supposed impossibility was realized. One may say that these 

 roots in Pennaria may give rise later on to polyps. Tn the special case that I 

 observed they did not, although as a rule they do. But the same is the case in 

 Tubularia, in which polyps also ari.se from the roots. It might be said, perhaps, 

 that the formation of roots in Pennaria is, for some reason, absolutely necessary. 

 But it is just as easy to produce polyps at both ends. Even if it were possible to 

 reconcile these facts with the principles of natural selection, causal or phys'o- 

 logical morphology would not gain thereby, as the circumstances that determine 

 the forms of animals and plants are only the different forms of energy in the 

 sense in which this word is used by the physicist, and have nothing to do with 

 natural selection. 



