32± THE LAW OF EVOLUTION. 



of the body; while in the more perfectly-formed annulose 

 animals, this double chain becomes united into a single 

 chain. Mr. Xewport has described the course of this con- 

 centration as exhibited in insects; and by Rathke it has been 

 traced in crustaceans. During the early stages of the ^4,9- 

 tacusfluviatilis, or common cray-fish, there is a pair of sepa- 

 rate ganglia to each ring. Of the fourteen pairs belonging 

 to the head and thorax, the three pairs in advance of the 

 mouth consolidate into one mass to form the brain, or ce- 

 phalic ganglion. Meanwhile, out of the remainder, the first 

 six pairs severally unite in the median line, while the 

 rest remain more or less separate. Of these six double 

 ganglia thus formed, the anterior four coalesce into one 

 mass; the remaining two coalesce into another mass; and 

 then these two masses coalesce into one. Here we see longi- 

 tudinal and transverse integration going on simultaneously; 

 and in the highest crustaceans they are both carried still fur- 

 ther. The Vertebrata clearly exhibit transverse integration 

 in the development of the generative system. The lowest 

 mammals — the Monotremata — in common with birds, to 

 which they are in many respects allied, have oviducts which 

 towards their lower extremities are dilated into cavities, 

 severally performing in an imperfect way the function of a 

 uterus. " In the Marsxijnalia there is a closer approximation 

 of the two lateral sets of organs on the median line ; for the 

 oviducts converge towards one another and meet (without 

 coalescing) on the median line; so that their uterine dilata- 

 tions are in contact with each other, forming a true ' double 

 uterus.' ... As we ascend the series of ' placental ' mam- 

 mals, we find the lateral coalescence becoming more and 

 more complete. ... In many of the Rodentia the uterus 

 still remains completely divided into two lateral halves; 

 whilst in others these coalesce at their lower portions, form- 

 ing a rudiment of the true ' body ' of the uterus in the 

 human subject. This part increases at the expense of the 

 lateral ' cornua ' in the higher herbivora and carnivora ; but 



