I borrowed this post's title from a Rudyard Kipling
Just So story title dealing with elephants, in case you might be curious.
For some reason hard to fathom 90 years and more later, American automobiles did not have storage areas worthy of the name.
For example, even this late-1920s Checker taxi in New York City has no roof rack or other obvious place to store luggage and other large objects. I suppose these were placed between the passengers' knees and the driver's seat.
Trunks became an industry-wide feature during the 1930s, the decade when car design evolution was at tis strongest. The focus here is on four-door sedans because coupés and some two-door sedans, due to their shorter passenger compartments, got integrated trunks sooner than their four-door siblings.
Examples of trunks from that era are shown below. Unless noted, images are of cars listed for sale on the internet.
Gallery
1929 Auburn 8-90
This Auburn is typical of its time, having no trunk.
1929 Hudson Club Sedan by Biddle & Smart - RM Sotheby's photo
However, some larger upscale cars did have trunks. This Hudson's trunk is conventional, being a separate item placed aft of the main body of the car.
1930 Stutz Monte Carlo, Weymann body - RM Sotheby's
This Stutz appears to have two trunks, a separate unit at the extreme rear as well as a trunk abaft of the passenger compartment accessible from outside. This was extremely rare in 1930 for four-door sedans.
1931 Marmon Sixteen Close-Coupled Sedan by LeBaron - Hyman Auctions photo
Marmon V-16 cars were among the those at the top of the price/prestige ladder, and this one has a trunk attached to the car's body. It is a separate element, not integrated like the Stutz's trunk in the previous image.
1933 Cadillac Town Sedan
By 1933 trunks like this were becoming common on larger cars.
1934 Studebaker Land Cruiser - my photo
Here is an example of an integral trunk several years ahead of its time. The design of this car was derived from the 1933 Pierce-Arrow Silver Arrow.
1935 Buick 41 Club Sedan
More typical is the trunk on this Buick. It remains visually distinct, yet its edges are smoothed into the main body -- a step towards complete integration.
1936 Cord 810 Beverly - Hyman
Standard Cord 810 Westchester sedans had a fastback design incorporating a lidded trunk. Beverly models had fancier interiors and some were given the tacked-on trunk lid seen here to provide some extra storage space.
1936 Lincoln Zephyr - factory photo
Aerodynamically-influenced Zephyrs has integral trunks.
1938 Studebaker Commander
Less radical is the trunk on this Studebaker. In some respects it's similar to that on the 1935 Buick above, but its blending into the body is more complete.
1938 Graham
Now we see an early version of the "bustle back" style common after the war.
1939 Mercury Eight
A "fastback" design like the Zephyr, but this Mercury's trunk is wider and more practical.
1941 Pontiac Torpedo Sedan
By the early 1940s integral trunks were the industry standard.