Fascinating photos show how drastically some popular dog breeds have changed over the past 100 years

Many well-known dog breeds have changed a lot physically in the last century, thanks to humans.

By identifying specific traits such as size, coat color, and demeanor and allowing only those animals to mate, humans have created at least 167 different "breeds," or groups of dogs with unique physical and mental characteristics. Still, they're all part of the same species.

The Science of Dogs blog put together a side-by-side comparison of several popular breeds from the 1915 book " Dogs of All Nations " by Walter Esplin Mason, showingwhat they look like today.

Here are some of the dogs from that list, plus a few more we found ourselves.

Tanya Lewis contributed to an earlier version of this post.

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Fascinating photos show how drastically some popular dog breeds have changed over the past 100 years

The bull terrier in 1915 appears to have been a fit dog with a well-proportioned head and slim torso. "Dogs of All Nations" called it the "gladiator of the canine race."

Todays bull terriers are bred to have a football-shaped head and a thick, squat body a shift from the lean and handsome dog of 1915.

Few dogs have been as artificially shaped by breeding as the English bulldog. In the UK, the dogs wereused for bull-baiting a blood sport where dogs were used to bait and attack bulls until it became illegal in 1835.

In 1915, thebulldogalready had some of the characteristic features we see today, like saggy jowls and a squat stance. But modern day bulldogs havemore pronounced facial wrinkles, and aneven thicker and squatter body.

In 1915, "Dogs of All Nations" described the German shepherd as a "medium sized dog" weighing just 55 pounds, with a "deep chest, straight back and strong loins."

But todays German shepherds are bred to be considerably larger 75 to 95 pounds with a more sloping back.

The American Kennel Club (AKC) describes the ideal specimen as "a strong, agile, well muscled animal, alert and full of life." These qualities may explain why the German shepherds place in the police force dates back a full century.

The German shepherd isnt the only dog whose role has stayed consistent for over 100 years. According to the AKC, pugs are among the oldest breeds of dogs, and they have served humans by keeping them company since the beginning of their time.

Although pugs have stayed roughly the same size over time, they have been selectively bred to have bigger, wider eyes and flatter noses, according to the Guardian.

Unfortunately, flatter faces, in which the nose is closer to the rest of the face, have been linked to breathing problems and other health risks.

Boston terriers also have significantly flatter faces than they did 100 years ago ...

... but, like pugs, these terriers have stayed roughly the same size in a century.

Aside from their flattening faces, French bulldogs are made to be shorter and stalkier than they used to be. The AKC describes them as "a muscular dog of heavy bone.

But the modern French bulldog still has some of the same features as it did 100 years ago ...

... like having a big head.

Though you cant tell from this photo, "Dogs of All Nations" described thecoloring of the Airedaleshead and ears asa rich tan, as well asthe legs up to the thighs and elbows. And thedogs coat was "hard and wiry," but not long enough to be "ragged"...

...until now. Today, the color appears not to have changed much, but the fur of modern Airedales appears to be longer and more "ragged" than it was in 1915.

The Shetland sheep dog has doubled in size since "Dogs of All Nations" reported that it weighed just 7 to 10 pounds and appeared to have medium-length fur.

Today, the dogs have been bred to be larger, weighing at least 20 pounds, though still slight. And their fur has become unmistakablylonger since 1915. The AKC now describes them as "small, alert, rough-coated, long-haired working" dogs.They are also very intelligent and good at herding.

Chow Chows, which used to weigh about 50 pounds, now weigh up to 75 pounds.

Some of this additional weight comes through in their faces, which are much more wrinkly than they used to be.

Leaner than the modern version, Great Danes used to weigh around 120 pounds, according to "Dogs of All Nations." Today, the large breed weighs more, with males reaching up to 175 pounds.

According to the AKC, Great Danes used to be used for hunting wild boars, but now they typically reside as the protectors of their homes.

Huskies have historically been sled dogs, but according to the AKC, sledding for function turned to sport as the breed gained popularity in the early 20th century.

Today, huskies have pointier ears than they used to, but they have always been known for their varying size and color.

Every pack of sled dogs has a leader, according to "Dogs of All Nations," and this husky was a record-setting sled dog that was labeled "one of the greatest leaders Alaska has ever known."

Newfoundlands are another breed of northern workers. Pictured here hauling a tree, the breed is historically full of lifeguards. Newfoundlands boarded ships with Canadian fisherman and rescued people from icy waters.

West Highland white terriers used to work as rat exterminators, according to the AKC.

But todays working white terrier, typically an inch or two shorter, lives a different kind of life the breed has become known more recently for modeling for brands including Juicy Couture and Black & White Scotch.

The Old English sheepdog is also known for its looks. These actor dogs have been casted in movies including Disneys "The Shaggy Dog" and "The Shaggy D.A.," according to the AKC.

But this hasnt always been the case. Despite its name, the shaggier 20th century Old English sheepdogs were historically used to herd cattle, according to the AKC.

Poodles started out as duck hunters and shifted the work to water retrieving about 400 years ago, according to the AKC. While they are known for their poofy, flamboyant hairstyle, this wasnt always the only popular poodle do.

A century ago, corded poodles were among the most popular of the breed. The flamboyant poof cut started out of function rather than form.

According to the AKC, owners began shaving poodles to make it easier for them to move around water, and they left hair on the areas that need protection from the cold.

Another breed known for its haircut, the Scottish terriers coat is much longer than it used to be, and the difference results in a different shape and texture.

Scottish terriers had two-inch coats 100 years ago. According to the AKC, longer coats are softer than shorter ones. Scottish terriers used to have wiry coats, according to "Dogs of All Nations."

The basset hound has essentially become a caricature of itself. Its somewhat-extreme characteristics have been exaggerated further over time.

Their long ears have gotten longer, their short legs have gotten shorter, and their elastic skin has gotten looser, according to the AKC.

Not all breeds have so drastically, however for the Great Pyrenees, the differences are in the details.

While the breed now has slightly larger eyes and slightly smaller ears, the line from outer eye to ear is a remaining characteristic, according to the AKC.

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