254 ANIMAL MORPHOLOGY 



fold aspect of form and function should be the object of his investiga- 

 tions their difference in allied species, their integrity, their behavior 

 in breeding, their phylogenetic origin, their embryological develop- 

 ment, and their maintenance in the adult. 



Morphology has relations with sciences quite outside of biology. I 

 have already insisted that the problems of form and structure are also 

 physiological problems, but in last analysis they are, I think, pro- 

 blems of physics and chemistry. For myself, I have no doubt that we 

 shall some day be able to prove that each characteristic of an organ- 

 ism depends upon a specific substance in the germ-cell, and we may 

 hope by altering this substance experimentally to change the corre- 

 sponding characteristic. Such a change is mutation, and mutation in 

 last analysis, as De Vries maintains, depends upon external condi- 

 tions. 



Apart from this it is certain that the physiological processes in- 

 volved in the individual's characteristics are modifiable, and, indeed, 

 controlled by physical agents in the environment. 1 Thus it has been 

 possible to show that certain salts play special roles in the develop- 

 ment of particular organs or characteristics (Herbst). Loeb, indeed, 

 has shown that regeneration of hydroids does not occur in the absence 

 of potassium. We know, likewise, that iron is necessary to the forma- 

 tion of the chromatin of the nucleus. 



The physical conditions have likewise an influence in morphogene- 

 sis. The rate of development is controlled within limits by tempera- 

 ture; the number and position of stomata and of leaves by light and 

 moisture; the number and form of plant hairs by moisture; the 

 position of branches and leaves on a stem by gravity ; the formation 

 of a hydranth in a hydroid stock by light. So evident is this depend- 

 ence of morphogenesis upon physical agents that two individuals 

 of the same family develop alike only under the same conditions of 

 environment. 



There remain to be considered the relations of morphology to the 

 queen of the sciences, to mathematics. Until recent years little 

 relation has been recognized, and this I attribute to the fact that few 

 naturalists have a type of mind that attracts them to mathematics. 

 They have usually been led to their science through a love of nature, 

 a passion that belongs rather to the poetic type of mind than to 

 the severely precise mathematical. And so I find that, even to-day, 

 when the bearing of mathematics on morphological problems cannot 

 be overlooked, few morphologists take an interest in the subject of 

 biometry by which the two sciences are connected. 2 



1 References to the literature on this general subject up to five or seven years 

 ago will be found in my Experimental Morphology, New York, vol. i, 1897; vol. n, 

 1899. 



2 For references on biometric subjects I may be permitted to refer to my Statis- 

 tical Methods, 2d ed., New York, 1904, which includes also a summary of results. 



