96 ARKIV FÖR ZOOLOGI. BAND 14. N:0 4. 



When the invasion of primitive Marsupials took place in 

 Australia, there were also certain Polyprotodonts among them 

 which became arboreal and in consequence of this developed 

 an opposable hallux and syndactylism. This happened inde- 

 pendently of tbe corresponding adaptation in the Caenolestes- 

 like Marsupials and is to be regarded as a case of convergent 

 development due to similar habits. 



The conditions of life were, however, gradually altered 

 in Australia. Probably this was due to a change to a more 

 dry climate so that the forests could not flourish as before. 

 Many of the arboreal forms bad then to change their habits 

 and try their struggle for existence on terra firma. So did 

 among the descendants of the Caenolestes-like forms, the 

 ancestors of the Kangaroos and among the offspring of pri- 

 mitive Polyprotodonts, the progenitors of Peramelidae. A 

 second time these groups of Marsupials changed their mode 

 of life in an analogous and parallel manner, the first time 

 from terrestrial to arboreal and now from arboreal to ter- 

 restrial. It is already stated that the first change brought 

 about an opposable hallux and syndactylism together with 

 a certain reduction of the second and third toes in both 

 groups. When the formerly arboreal animals had to move 

 about on earth again the hallux had a position which made 

 it less useful or even cumbersome and it was thus reduced, 

 but the united and dwarfed second and third toes could not 

 regain their former strength so that the fourth became the 

 main supporting toe. The members of both groups under- 

 went for the second time analogous structural alterations, 

 because their organisation with respect to their feet, had 

 been similar, and the new change of life, alike for both, had 

 the same influence on both. Thus according to my opinion 

 the syndactylism etc. of Macropodidae and Peramelidae is a 

 product of paralell development caused by the influence of 

 similar habits of life, but the resemblance between the inci- 

 sors of Caenolestes on one side and the kangaroos etc. on 

 the other is due to inheritance and genetic affinity leading 

 back to common ancesters, which already decidedly had 

 taken the first irrevocable and unreturnable steps towards 

 Diprotodont development. 



