The use of BECOME in Jack London's The Call Of The Wild

Far more potent were the memories of his heredity that gave things he had never seen before a seeming familiarity; the instincts (which were but the memories of his ancestors become habits) which had lapsed in later days, and still later, in him, quickened and become alive again.

Hello guys, I want to figure out the reason why Jack London used the verb 'become' in its present form in the part of the quoted sentence that lies within the brackets : which were but the memories of his ancestors become habit.

From my reading, I had expected it to be modified into 'which were but the memories of his ancestors becoming habits." Only by that way could it fit into into my understanding of this great quote. But I always came to realize that it was just a shortcoming in my own grammar skills that prevented me from understanding Jack London's writing.

Please help me analyse the wording of the above sentence. Thanks a lot!

P/S: Also, in my kindle free edition of The Call Of The Wild, the last 'become' near the end of the sentence is also in its present form, not 'became'. This stands odd besides 'quickenED' and thus puzzles me a lot.

Solution 1:

It's not really a tense shift - it's more an abbreviated and elaborated sentence style, which London uses a lot.

In the full form :

(which were but the memories of his ancestors [which had] become habits)

and, abbreviating the second example further,

the instincts ... which had ... become alive again.

Past tense ("which had become") in both cases, but the abbreviation in the first instance and the intervening thoughts in the second have separated the relevant parts of the verb.