£52 M. O'Connell — Orthogenetic Development 



that the term orthogenesis has been used by some writers 

 to describe certain observed phenomena and by others 

 to explain the origin of those phenomena. It is for this 

 very reason that so much confusion has arisen and many 

 who cannot see in orthogenesis a cause of evolution deny 

 its existence as a mode of development. If at the outset, 

 then, we distinguish between the fact or law of ortho- 

 genesis and a possible, and as yet undiscovered, cause of 

 orthogenesis, the matter is greatly simplified and we lay 

 claim to nothing but what we can see. It is true that 

 Eimer believes that he has discovered the cause of ortho- 

 genesis, but his statements are open to criticism and 

 there is no question that his data and observations are 

 capable of other interpretations than the ones which he 

 has given. What we cannot deny, however, is that he 

 did bring together many excellent illustrations of the law 

 of orthogenesis whether or not we believe in the expla- 

 nation which he offered. 



Considering, then, that orthogenesis is to be applied 

 to facts, to observable phenomena, we must make one 

 further restriction and state that the observations made 

 must be of successive changes, either in the growth of 

 an individual, that is, in ontogeny, or in the evolution of 

 the race, that is, in phylogeny. This idea of succession 

 is bound up in the very definition of orthogenesis and 

 must not be lost sight of. 



There are four fields in which observations may be 

 made to a greater or less extent in carrying on ortho- 

 genetic studies, namely, those of experimental and obser- 

 vational biology, embryology, vertebrate paleontology 

 and invertebrate paleontology. 



It- is at once apparent from our restricted definition 

 of orthogenesis that the zoologist seldom has the facts 

 of orthogenesis presented to him in his studies, for he 

 deals with individual adults, in which he sees only single 

 isolated stages in the ontogeny, and he deals with living 

 forms, having thus nothing to do with phylogeny, except 

 in so far as this is presented in a single chronofauna. 

 The experimental zoologist, or genetecist, concerns him- 

 self neither with successive changes in the development 

 of the individual nor with the changes through geologic 

 time in species, genera or phyla and he, therefore, never 

 has presented to him the data for orthogenetic studies. 

 The observational biologist may, to a limited degree, 



