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R. Ruggles Gates. 
organism can, as it were assert a power over the cells in maintain¬ 
ing a unity of structure during development to a degree which is 
not possible with the thicker cellulose walls of plants, though in 
woody stems growth-pressure produces a certain amount of such 
readjustment of dead cells. In any case, whatever the reason, it 
remains true that while development in plants is usually direct, 
and recapitulation the exception, the animal embryologist is 
confronted with recapitulation on all hands with such amazing 
profusion that a comprehensive principle is more obviously required 
for their explanation. 
Another explanation of the difference between plants and 
animals in the occurrence of recapitulation may lie in a greater 
frequency of mutations in the phylogeny of plants. Also, animal 
groups in their evolution have probably passed more frequently by 
adaptation from one habitat to another. Witness, for example, the 
number of groups from Ephemeridae to whales or penguins which 
have become secondarily modified for an aquatic life. Even when 
a like occurrence happens in plants, the stages of it are often nearly 
or quite obliterated by short-circuiting. It is therefore quite unsafe 
to argue in the case of plants that because a given adaptational 
character shows no recapitulation in development it must have 
originated through a germinal change. Such an attitude stands 
a better chance of being sound in the case of animals. 
Concerning the facts of recapitulation, MacBride’s textbook of 
embryology—Invertebrata (1914) is a veritable mine of information. 
His attitude represents a return to an interpretation of the 
significance of larval stages in relation to phylogeny on a frankly 
neo-Lamarckian basis. Embryologists of the last two decades have 
largely endeavoured to avoid this attitude, but without conspicuous 
success. MacBride assumes that larval stages represent actual 
ancestral groups of organisms. It is also significant, as we pointed 
out in the case of plants, that the recapitulation seems always to 
have originated in connection with the adaptation of the animal to 
a new set of conditions. 
Although we have never seen it explicitly stated that embryonic 
recapitulation implies the inheritance of acquired characters, 1 yet 
it is probably the tacit recognition of this fact which has led to the 
denial of recapitulation by those who believe only in germinal 
variations as material for evolution. 
1 Since this was written I have received from Professor MacBride (1917) 
a paper which I had not previously seen, in which it is definitely stated that 
recapitulation implies the inheritance of acquired characters, and citing a 
number of cases in support of this view. 
