18 HYATT ON THE TERTIARY SPECIES 



individual, and similar correspondences be traced in both, and also that the tendencies 

 exhibited are of two kinds in each, one towards a building up of the organization, and the 

 other directly opposed to this. 



We cannot understand these remarkable concordances between the changes shown in 

 the succession of the forms in allied or genetic series, and the metamorphoses of the indi- 

 vidual, unless we can attribute them to a similar cause. The fact, that during the growth 

 of the individual, the increase in bulk, and all increments, whatever be their nature, must 

 be due to an excess in the supply of nutrimjent over and above what is needed to repair 

 the waste of the body, alone shows that there is a force at work within the organiza- 

 tion. The action of this innate power of the organism therefore is fundamental, and hes 

 at the base of all changes whatever, except the strictly retrogressive, and it becomes evi- 

 dent that the same force which causes growth also occasions all progressive changes. 



It is sometimes the case that, as in the third sub-series, the force described produces a 

 series of progressive characteristics without increasing the size ; but this, and also the very 

 frequent decrease in bulk of full grown animals when compared with their larval forms, as 

 in many butterflies and moths, etc., and in some frogs, may be accounted for by the greater 

 development of functional activity. The phenomenon is similar to the stunting of the 

 growth of an animal by the too early and powerful use of its organs. The growth power is 

 used up in the assimilation of the formative material for the new parts produced, and their 

 functional waste is so great that there is no material for increase in bulk. 



If we apply the presence of this unknown power or force within the animal to the 

 explanation of the characteristics of the series, we are struck by the ease with which all 

 the phenomena of parallelisms are resolved. By reference to the laws of growth and 

 development the naturalist is able to explain why it is that all the forms and modifica- 

 tions on the progressive grades, show similarities to the metamorphoses of the growing 

 individuals ; why all the forms of the descending grades compare so closely with the senile 

 metamorphoses of the individual ; why it is that some series, like some individuals, have no 

 perceptible and others have a very complete series of old age metamorphoses ; why all the 

 forms and characteristics of the progressive series succeed each other in similar order in 

 every series, occasioning the reproduction in each series of an independent but similar 

 parallel series of forms and characteristics ; why it is that there is a similar succession in 

 the development of the characteristics in each representative species ; why it is that differ- 

 ent tendencies, one towards the production of progressive, and the other towards the 

 production of retrogressive characteristics, can appear in the retrogressive series in 

 different combinations or quantities according to the series; why it is in fact that the 

 whole series of modifications in the group and its series, can be approximately com- 

 pared with the life of one individual. The uniformities of series of animals as compared 

 with each other then become like the uniformities observable in the growth of closely 

 allied individuals, like their parallelisms in growth and in old age or disease, all due to 

 the force of the inheritable constitution, enabling the animal to take advantage of favorable 

 surroundings for a time, or to resist the effects of its unfavorable environment more or less 

 successfully. In all cases the individuals and its series must change by growth along 

 certain lines of modifiation, which it is but reasonable to suppose we shall some day be 

 able to map out beforehand for a series of forms with the same precision that we can 

 now forecast the metamorphoses of any given individual in a given species. 



