THE GASTRAEA-THEORV, ETC. 153 



discovered are strong proofs of the truth of theGastraea-theory. 

 This may also be said of tlie remarkable and valuable investi- 

 gations on the ontogenesis of the lower animals which Edouard 

 van Beneden, jun., has published in several memoirs, es])eci- 

 ally in his prize essay on the composition and significance of 

 the animal ovum^ (1870.) 



E. Ray Lankester has lately published (in May, 1873) 

 an essay well worth reading on the primitive germ-lamellse, 

 and their significance in the classification of the animal 

 kingdom.^ which is in substantial agreement with the suc- 

 cession of ideas which have led me on to the Gastraea- 

 theory. It is true that, in particulars, we in different ways 

 diverge, and especially different are our views of the secon- 

 dary germ-lamellae, as well as of the coelom, and of the 

 relation of the vascular system with the primitive segment 

 organs. But in most respects, and especially with regard to 

 the homologies of the primary germ-lamellse, Ray Lankester's 

 ideas completely agree with mine. This agreement is so 

 much the more satisfactory, as we have been working inde- 

 pendently of each other, and have arrived at the same result 

 by different methods. 



In regard to the conclusions which I subsequently draw 

 from the Gastraea-theory, and some of which affect the 

 most important principles of comparative anatomy and de- 

 velopmental history, as well as the classification of the animal 

 kingdom, I must lay claim to that liberty of natural philo- 

 sophical speculation (or in other words, intelligent com- 

 parison of empirical results), without which, in my opinion, 

 general biology cannot advance a step forwards. I have 

 fully explained my ideas of the right of necessarily combin- 

 ing the empirical and philosophical methods in my " critical 

 and systematic introduction to the general morphology of 

 organisms," as well as in my systematic introduction to the 

 monograph of calcareous sponges, and can here simply refer 

 to these detailed justifications for ray adopting this stand- 

 point. 



In any case, proof may be given by the following disqui- 

 sition, that Cuvier's and Baer's theory of types which has 

 formed the basis of zoological classification for more, up 

 to the present day, than half a century, has been rendered 

 untenable by the progress of ontogenesis. In its place, the 



1 Edouard van Benedeu, ' lleclierches sur la composition et la signification 

 de I'ceuf,' Bruxelles, 1870. 



" E. Ray Laiikester "On the Primitive Cell-layers of the Embryo as tlie 

 Basis of Genealogical Classification of Animals, and on the Origin of Vascular 

 and Lymph Systems j" 'Annals and Mag. of Natural History,' 1873, vol. xi, 

 p. 321. 



